Exhaust gas purifying system

ABSTRACT

An exhaust gas purifying system for an internal combustion engine mounted on an automotive vehicle. The exhaust gas purifying system comprises an exhaust gas component concentration regulating device disposed in an exhaust gas passageway of the engine, for regulating concentrations of gas components in exhaust gas discharged from the engine such that the concentrations of carbon monoxide and hydrogen are respectively not more than 2.0% by volume and not less than 0.5 by volume and such that a volume concentration ratio of [hydrogen/carbon monoxide] is not smaller than 0.5, in rich and stoichiometric conditions of exhaust gas. Additionally, a NOx adsorbing and reducing catalyst is disposed in the exhaust gas passageway downstream of the exhaust gas component concentration regulating device, for adsorbing nitrogen oxides in a lean condition of the exhaust gas and reducing the nitrogen oxides into nitrogen in the rich and stoichiometric conditions.

[0001] This application is a continuation-in-part of U.S. application Ser. No. 09/948,876, filed Sep. 10, 2001.

BACKGROUND OF THE INVENTION

[0002] This invention relates to improvements in an exhaust gas purifying system for purifying exhaust gas discharged from an internal combustion engine (gasoline-fueled or diesel) of an automotive vehicle or a boiler, regarding noxious components such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx), and more particularly to the exhaust gas purifying system for effectively removing NOx, HC and CO in an oxygen-excessive region (lean region) or a low temperature region of exhaust gas.

[0003] In recent years, automotive vehicles of a low fuel consumption have been eagerly desired from the view points of exhaustion of petroleum resource and warming-up phenomena of the earth. Regarding automotive vehicles with a gasoline-fueled engine, attention has been paid on development of automotive vehicles provided with a so-called lean-burn engine. In the automotive vehicles provided with the lean-burn engine, exhaust gas is in an oxygen-excessive or lean region (atmosphere) in which an air-fuel (air/fuel) ratio is leaner or larger than a stoichiometric value. In case that a usual three-way catalyst is used in the lean region, removing or reducing NOx can be insufficient under the effect of the excessive oxygen. Accordingly, development of a catalyst for effectively reducing NOx even under an oxygen-excessive condition has been desired.

[0004] A catalyst for reducing NOx in such a lean region has been proposed as disclosed in Japanese Patent Provisional Publication No. 5-168860 in which Pt and lanthanum are carried on a porous carrier so that NOx is adsorbed in the lean region and released in a stochiometric region in which exhaust gas has a stoichiometric air-fuel (air/fuel) ratio. However, there is the problem that a sufficient NOx reducing effect cannot be obtained even if such a catalyst is used.

[0005] Otherwise, the three-way catalyst has hitherto been used to simultaneously carry out oxidation of CO and HC and reduction of NOx in exhaust gas from a gasoline-fueled engine. A representative example of such a three-way catalyst is produced by forming a carrier layer of y-alumina on a heat-resistant substrate of cordierite or the like, and by causing catalytic noble metals such as platinum (Pt), palladium (Pd) and rhodium (Rh) to be carried on the carrier layer.

[0006] Now, from the viewpoint of protection for global environment, carbon dioxide (CO₂) in exhaust gas discharged from the internal combustion engine of an automotive vehicle or the like is problematic. In view of this, the lean-burn engine for accomplish lean-burn in the lean region and diesel engine seem to be full of promise. These engines can improve the fuel consumption or economy of the automotive vehicle under the effect of reduction of fuel to be consumed thereby suppressing generation of CO₂ as a combustion or exhaust gas from the engine. Such a catalyst for removing HC, CO and NOx in the lean region is disclosed in Japanese Patent Provisional Publication No. 9-57098. In this catalyst, Pt and Rh are carried separate from Pd so that NOx is reduced under the action of Pt upon being adsorbed or trapped in a NOx adsorbing or trapping material while HC and CO in a stochiometric or rich region is oxidized under the action of Pd.

[0007] However, the above NOx adsorbing material is not advantageous for adsorption or trap of NOx in a considerably low temperature region (not higher than 140° C.), and therefore there is the possibility of lowering the absorption amount of NOx. Additionally, it has been known that the catalytic activity of the catalyst varies according to the kinds of the catalytic noble metals to be used. Additionally, Pd tends to be easily poisoned with sulfur compounds existing in exhaust gas, as compared with Pt. Further, it has been known that NO, HC and the like coexisting in exhaust gas impede the catalytic activity for oxidation of CO in the considerably low temperature region of exhaust gas.

[0008] By the way, it is conventional that diesel engines or the like are equipped with a catalyst disposed in an exhaust gas passageway so as to remove or reduce NOx discharged from the engine. Known examples of such a catalyst are zeolite-based one, alumina-based one and the like; however, they can exhibit a NOx reducing effect only within a limited temperature range. In view of this, it has been proposed to combine a plurality of catalysts which have different temperature ranges in which the NOx reducing effect can be exhibited, for the purpose of extending the temperature range exhibiting the NOx reducing effect. For example, Japanese Patent Provisional Publication No. 6-134258 discloses an exhaust gas purifying system which includes a plurality of catalysts which have different temperatures at which the maximum catalytic activities are obtained, upon varying the amount of cobalt carried on mordenite of the catalysts. It is usual to dispose the catalyst having a relatively high temperature activity on the upstream-side of the catalyst having a relatively low temperature activity in the exhaust gas passageway. Additionally, Japanese Patent Provisional Publication No. 6-307231 discloses an exhaust gas purifying system including a plurality of NOx reducing catalysts which are arranged in series relative to flow of exhaust gas and such that their oxidation activity for hydrocarbons increases as their location shifts to the downstream-side.

[0009] Thus, although the temperature range exhibiting the NOx reducing effect of the exhaust gas purifying system can be extended by combining a plurality of NOx reducing catalysts, a NOx reducing efficiency is low at a part where curves of the NOx reducing effects of the plural NOx reducing catalysts cross, forming a trough in NOx reducing efficiency within a temperature range. Accordingly, even by combining the plural NOx reducing catalysts, a practically sufficient NOx reducing efficiency cannot be obtained for the reason of existence of the NOx reducing efficiency trough within the temperature range, which is largely problematic. Particularly within a low temperature range at which the catalytic activity of the catalysts is inherently low, the amounts of NOx to be reduced by the catalysts are small even upon using the plural catalysts, and therefore the NOx reducing efficiencies of the catalysts are sharply lowered.

[0010] In view of the above background, the present inventors have found such a knowledge that a multistage catalyst system cannot achieve a high improvement in NOx reducing efficiency, because reducing agents (such as CO and HC) highly useful in NOx reduction in a high temperature range impede NOx reduction in a low temperature range.

SUMMARY OF THE INVENTION

[0011] It is, therefore, an object of the present invention to provide an improved exhaust gas purifying system for an internal combustion engine, which can overcome drawbacks encountered in conventional exhaust gas purifying systems including catalysts for purifying exhaust gas.

[0012] Another object of the present invention is to provide an improved exhaust gas purifying system for an internal combustion engine, which can effectively remove or reduce NOx in exhaust gas in an oxygen-excessive or lean region, discharged from the engine by regulating the concentrations of the gas components of exhaust gas.

[0013] A further object of the present invention is to provide an improved exhaust gas purifying system for an internal combustion engine, which can effectively remove or oxidize CO and HC in exhaust gas even in a low temperature region by regulating adsorption and release of NOx in accordance with the temperature of exhaust gas.

[0014] A still further object of the present invention is to provide an improved exhaust gas purifying system for an internal combustion engine, which can effectively remove or reduce NOx in exhaust gas in an oxygen-excessive or lean region, discharged from the engine by regulating the concentrations of the gas components of exhaust gas, while effectively removing or oxidizing CO and HC in exhaust gas even in a low temperature region by regulating adsorption and release of NOx in accordance with the temperature of exhaust gas.

[0015] A still further object of the present invention is to provide an improved exhaust gas purifying system for an internal combustion engine, which can exhibit a high NOx removing efficiency throughout a wide range of exhaust gas temperature, particularly in a low exhaust gas temperature range.

[0016] An aspect of the present invention resides in an exhaust gas purifying system for an internal combustion engine, comprising an exhaust gas component concentration regulating device disposed in an exhaust gas passageway of the engine, for regulating concentrations of gas components in exhaust gas discharged from the engine such that the concentrations of carbon monoxide and hydrogen are respectively not more than 2.0% by volume and not less than 0.5 by volume and such that a volume concentration ratio of [hydrogen/carbon monoxide] is not smaller than 0.5, in a first exhaust gas condition in which air/fuel ratio of exhaust gas is within a range of from a rich value and a stoichiometric value. Additionally, a NOx adsorbing and reducing catalyst is disposed in the exhaust gas passageway downstream of the exhaust gas component concentration regulating device, for adsorbing nitrogen oxides in a second exhaust gas condition in which the air/fuel ratio of exhaust gas is at a lean value, and reducing the nitrogen oxides into nitrogen in the first exhaust gas condition.

[0017] Another aspect of the present invention resides in an exhaust gas purifying system for an internal combustion engine, comprising a NOx adsorbing catalyst disposed in an exhaust gas passageway of the engine, the NOx adsorbing catalyst adapted to adsorb and release nitrogen oxides in accordance with a temperature of exhaust gas, the NOx adsorbing catalyst adapted to adsorb nitrogen oxides in exhaust gas in a first exhaust gas temperature range in which exhaust gas has a temperature ranging from a level at engine starting to 140° C. and release the adsorbed nitrogen oxides in a second exhaust gas temperature range in which exhaust gas has a temperature of not lower than 200° C. Exhaust gas discharged from the NOx adsorbing catalyst has a volume concentration ratio of [nitrogen oxides/carbon monoxide] which is not larger than 0.3 in the first exhaust gas temperature range. Additionally, an oxidizing catalyst is disposed in the exhaust gas passageway downstream of the NOx adsorbing catalyst, for oxidizing oxidizable components in exhaust gas discharged from the NOx adsorbing catalyst.

[0018] A further aspect of the present invention resides in a NOx adsorbing catalyst used in an exhaust gas purifying system for an internal combustion engine. The NOx adsorbing catalyst disposed in an exhaust gas passageway of the engine and adapted to adsorb and release nitrogen oxides in exhaust gas in accordance with a temperature of exhaust gas. The exhaust gas purifying system includes an oxidizing catalyst in the exhaust gas passageway downstream of the NOx adsorbing catalyst, for oxidizing oxidizable components in exhaust gas discharged from the NOx adsorbing catalyst. In the above exhaust gas purifying system, the NOx adsorbing catalyst is adapted to adsorb nitrogen oxides in exhaust gas in a first exhaust gas temperature range in which exhaust gas has a temperature ranging from a level at engine starting to 140° C. and release the adsorbed nitrogen oxides in a second exhaust gas temperature condition in which exhaust gas has a temperature of not lower than 200° C. Exhaust gas discharged from the NOx adsorbing catalyst has a volume concentration ratio of [nitrogen oxides/carbon monoxide] which is not larger than 0.3 in the first exhaust gas temperature range.

[0019] A still further aspect of the present invention resides in an exhaust gas purifying system for an internal combustion engine, comprising an exhaust gas component regulating device disposed in an exhaust gas passageway of the engine, at least one of a NOx adsorbing and reducing catalyst and a NOx adsorbing catalyst disposed in the exhaust gas passageway downstream of the exhaust gas component regulating device, and an oxidizing catalyst in the exhaust gas passageway downstream of the at least one of the NOx adsorbing and reducing catalyst and the NOx adsorbing catalyst. In the above exhaust gas purifying system, the exhaust gas component concentration regulating device is adapted to regulate concentrations of gas components in exhaust gas discharged from the engine such that the concentrations of carbon monoxide and hydrogen are respectively not more than 2.0% by volume and not less than 0.5% by volume and such that a volume concentration ratio of [hydrogen/carbon monoxide] is not smaller than 0.5, in a first exhaust gas condition in which air/fuel ratio of exhaust gas is within a range of from a rich value and a stoichiometric value; the NOx adsorbing and reducing catalyst is adapted to adsorb nitrogen oxides in a second exhaust gas condition in which the air/fuel ratio of exhaust gas is at a lean value, and reducing the nitrogen oxides into nitrogen in the first exhaust gas condition; the NOx adsorbing catalyst is adapted to adsorb and release nitrogen oxides in accordance with a temperature of exhaust gas, the NOx adsorbing catalyst adapted to adsorb nitrogen oxides in exhaust gas in a first exhaust gas temperature range in which exhaust gas has a temperature ranging from a level at engine starting to 140° C. and release the adsorbed nitrogen oxides in a second exhaust gas temperature range in which exhaust gas has a temperature of not lower than 200° C., exhaust gas discharged from the NOx adsorbing catalyst having a volume concentration ratio of [nitrogen oxides/carbon monoxide] which is not larger than 0.3 in the first exhaust gas temperature range; and the oxidizing catalyst is adapted to oxidize oxidizable components in exhaust gas discharged from the at least one of the NOx adsorbing and reducing catalyst and the NOx adsorbing catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a schematic illustration of a first embodiment of an exhaust gas purifying system for an internal combustion engine, according to the present invention, having two catalysts:

[0021]FIG. 2 is a schematic illustration of a second embodiment of the exhaust gas purifying system for an internal combustion engine, according to the present invention, having two catalysts;

[0022]FIG. 3 is a schematic illustration of a further embodiment of the exhaust gas purifying system for an internal combustion engine, according to the present invention, having three catalysts;

[0023]FIG. 4 is a schematic illustration of a still further embodiment of the exhaust gas purifying system for an internal combustion engine, according to the present invention, having three catalysts;

[0024]FIG. 5 is a schematic illustration of a still further embodiment of the exhaust gas purifying system for an internal combustion engine, according to the present invention, having four catalysts;

[0025]FIG. 6A is an illustration of a modified example of the first embodiment of the exhaust gas purifying system according to the present invention;

[0026]FIG. 6B is an illustration of another modified example of the first embodiment of the exhaust gas purifying system according to the present invention;

[0027]FIG. 6C is an illustration of a further modified example of the first embodiment of the exhaust gas purifying system according to the present invention;

[0028]FIG. 7A is a flowchart of temperature control in the modified examples of FIGS. 6A to 6C;

[0029]FIG. 7B is a graph showing the switching states of an electric source for a heater used in the modified examples of FIGS. 6A to 6C in terms of inlet temperature of a upstream-side NOx adsorbing and reducing catalyst in the modified examples;

[0030]FIG. 8A is an illustration of a still further modified example of the first embodiment of the exhaust gas purifying system according to the present invention;

[0031]FIG. 8B is an illustration of a still further modified example of the first embodiment of the exhaust gas purifying system according to the present invention;

[0032]FIG. 9 is a flowchart of temperature control in the modified examples of FIGS. 8A and 8B;

[0033]FIG. 10 is a concrete illustration of the exhaust gas purifying system corresponding to that of FIG. 1;

[0034]FIG. 11 is a concrete illustration of the exhaust gas purifying system corresponding to that of FIG. 2; and

[0035]FIG. 12 is a concrete illustration of the exhaust gas purifying system corresponding to that of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

[0036] A first embodiment of an exhaust gas purifying system according to the present invention will be discussed hereinafter with reference to FIG. 1.

[0037] The exhaust gas purifying system is for an internal combustion engine and comprises an exhaust gas component concentration regulating device disposed in an exhaust gas passageway of the engine, for regulating concentrations of gas components in exhaust gas discharged from the engine such that the concentrations of carbon monoxide and hydrogen are respectively not more than 2.0% by volume and not less than 0.5% by volume and such that a concentration ratio (volume) of [hydrogen (H₂)/carbon monoxide (CO)] is not smaller than 0.5, in a first exhaust gas condition in which air/fuel ratio of exhaust gas is within a range of from a rich value and a stoichiometric value. Additionally, a NOx adsorbing (trapping) and reducing catalyst disposed in the exhaust gas passageway downstream of the exhaust gas component concentration regulating device, for adsorbing nitrogen oxides in a second exhaust gas condition in which the air/fuel ratio of exhaust gas is at a lean value, and reducing the nitrogen oxides into nitrogen in the first exhaust gas condition. A concrete example of this embodiment is illustrated in FIG. 10. A major part of the exhaust gas passageway is formed in an exhaust pipe through which exhaust gas from the engine flows to the NOx adsorbing and reducing catalyst. The engine is, in this instance, for an automotive vehicle. The rich value means an air/fuel ratio which is richer (in fuel) or smaller than the stoichiometric value. The lean value means an air/fuel ratio which is leaner (in fuel) or larger than the stoichiometric value.

[0038] As a result of study of the present inventors on reactions of from adsorbed NOx to N₂, the principle of exhaust gas purification of the present invention has been derived from the findings that H₂ can promote the most effectively the reactions and also from the finding that promotion of the reactions under the action of H₂ cannot be sufficiently made if a noble metal is covered with CO. In other words, NOx can be effectively removed or reduced by reducing the concentration of CO while maintaining a certain concentration of H₂ at a location (in the exhaust gas passageway) immediately upstream of a catalyst for reducing NOx.

[0039] In addition, regulating the above concentrations and concentration ratio of carbon monoxide and hydrogen to be flown to the NOx adsorbing and reducing catalyst has been derived from the following findings as a result of further study of the present inventors on the basis of the above findings: In case that the concentration of carbon monoxide is not more than 2.0% by volume, a reaction hindrance due to CO can be reduced. In case that the concentration of H₂ is not less than 0.5% by volume, a conversion efficiency of NOx to N₂ becomes sufficient. Additionally, a balance between the reaction hindrance due to CO and the reaction promotion due to H₂ becomes appropriate when the concentration ratio of [H_(2/)CO] is not smaller than 0.5.

[0040] In general, the concentration ratio between H₂ and CO in exhaust gas discharged from an automotive internal combustion engine is determined according to a thermal equilibrium in a water gas shift reaction represented by the following reaction formula (1):

CO+H₂O→CO₂+H₂  (1)

[0041] The concentration ratio of [H₂/CO] is usually about 0.3. This concentration ratio can be raised by disposing the above exhaust gas component concentration regulating device in the exhaust gas passageway, thereby improving the conversion (removal) efficiency of NOx in the NOx adsorbing and reducing catalyst located downstream of the exhaust gas component concentration regulating device.

[0042] The NOx adsorbing and reducing catalyst is adapted to adsorb (trap) NOx during a lean operation of the engine and to reduce the adsorbed NOx into N₂ during rich or stoichiometric operation of the engine. In the lean operation, the engine is operated on an air-fuel mixture having an air/fuel ratio leaner (larger) than a stoichiometric value so as to discharge exhaust gas having an air/fuel ratio leaner (larger) than the stoichiometric value. In the rich operation, the engine is operated on an air-fuel mixture having an air-fuel ratio richer (smaller) than the stoichiometric value so as to discharge exhaust gas having an air/fuel ratio richer (smaller) than the stoichiometric value. In the stoichiometric operation, the engine is operated on an air-fuel mixture having an stoichiometric air/fuel ratio so as to discharge exhaust gas having a stoichiometric air/fuel ratio.

[0043] The NOx adsorbing and reducing catalyst contains noble metals such as Pt, Pd and/or Rh (including any combinations of the noble metals), and a porous carrier or substrate carrying the noble metals, such as alumina and/or the like. The NOx adsorbing and reducing catalyst further contains alkali metals and/or alkaline earth metals such as sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and/or the like (including any combinations of the alkali metals and alkaline earth metals). It is preferable that the NOx adsorbing and reducing catalyst has a function serving as a usual three-way catalyst, and therefore may contain ceria, zirconia, lanthanum and/or the like which are usually employed in a three-way catalyst. In case that the NOx adsorbing and reducing catalyst has a function as a three-way catalyst, exhaust gas can be effectively purified in a condition (such as during engine acceleration) where the engine is operated at a high engine load on a stoichiometric air-fuel mixture.

[0044] During the lean operation of the engine, the air/fuel ratio of the air-fuel mixture to be supplied to the engine is preferably within a range of from 20 to 50 in which an adsorbing (trapping) reaction of NOx to the NOx adsorbing and reducing catalyst occurs at a high efficiency. During the rich operation of the engine, the air/fuel ratio of the air-fuel mixture is preferably within a range of from 1.0 to 14.7 in which the adsorbed NOx is converted into N₂ at a high efficiency in the NOx adsorbing and reducing catalyst.

[0045] In the exhaust gas purifying system of the present invention, the efficiency of conversion of NOx to N₂ becomes high as a temperature of exhaust gas at a location (in the exhaust gas passageway) immediately upstream of the NOx adsorbing and reducing catalyst is low. The conversion efficiency is particularly high at the exhaust gas temperature ranging from 150 to 250° C. In conventional catalysts, the reaction hindrance due to covering noble metal with CO is predominant as exhaust gas temperature is low. However, according to the exhaust gas purifying system of the present invention, the concentration of CO is regulated by the exhaust gas component concentration regulating device, so that exhaust gas low in CO concentration is flown into the NOx adsorbing and reducing catalyst on the downstream side, thereby reducing NOx into N₂ at a high efficiency.

[0046] Additionally, it is better that the concentration ratio of [H₂/CO] is as large as possible and set at a value of not smaller than 0.5, preferably not smaller than 1.0. This has been derived from the present inventor's knowledge that the reaction hindrance due to CO can be reduced when the concentration of CO is lowered whereas the conversion efficiency of NOx to N₂ can become sufficient when the concentration of H₂ is raised.

[0047] As the exhaust gas component concentration regulating device, there are a variety of devices, for example, a so-called rich exhaust gas CO and H₂ regulating catalyst for regulating the concentration of CO and H₂ in rich exhaust gas which has the air/fuel ratio richer (smaller) than the stoichiometric value. In concrete, an example of the rich exhaust gas CO and H₂ regulating catalyst is a shift reaction catalyst for promoting a water gas shift reaction represented by the following chemical formula (1):

CO+H₂O→CO₂+H₂  (1)

[0048] Under the action of the shift reaction catalyst, the reaction of oxidizing CO at the left side of the above formula (1) into CO₂ can effectively occur, exhaust gas having a large [H₂/CO] concentration ratio can be obtained. It is preferable that the shift reaction catalyst contains Pt, Rh and Ce (cerium) compound. It has been known that combination of a noble metal and ceria is advantageous for promoting the water gas shift reaction represented by the formula (1). In the rich exhaust gas CO and H₂ regulating catalyst, it is preferable that Pt and Rh of noble metals are used in combination with ceria, which is based on the present inventors' knowledge. Additionally, it is also preferable that the rich exhaust gas CO and H₂ regulating catalyst has a such a structure that a part of Pt and/or Rh is directly carried on the ceria, or activated alumina carries thereon ceria on which Pt and/or Rh are carried. The ceria preferably has a high heat resistance and may be in the form of complex oxide including zirconia, lanthanum and/or the like. Additionally, Pd may be added to the ceria in order to improve a low temperature activity of a three-way catalyst. The cerium is carried in an amount (calculated as cerium oxide) ranging preferably from 5 to 100 g, more preferably from 10 to 50 g per one liter of a substrate such as a honeycomb-type monolithic substrate. The substrate may be a pellet-type substrate.

[0049] Another example of the rich exhaust gas CO and H₂ regulating catalyst is a so-called H₂ low consumption catalyst which functions to consume a larger amount of CO than that of H₂ in the rich exhaust gas. By suppressing the consumption of H₂, exhaust gas having a [H₂/CO] concentration ratio larger than that in conventional exhaust gas purifying catalysts can be flown into the NOx adsorbing and reducing catalyst. The H₂ low consumption catalyst preferably contains, for example, compound(s) of Pt and Rh, and compound(s) of titanium and/or zirconium. Upon addition of titanium and/or zirconium, an oxidation reaction of CO can be promoted while oxidation of H₂ cannot be promoted as compared with the oxidation of CO, thereby providing exhaust gas having a large [H₂/CO] concentration ratio. It is preferable that titanium and/or zirconium is carried on a substrate preferably in an amount ranging preferably from 5 to 100 g and more preferably from 10 to 50 g (calculated as oxide) per one liter of the substrate.

[0050] A further example of the rich exhaust gas CO and H₂ regulating catalyst is a so-called CO decreasing catalyst for decreasing the concentration of CO in the rich exhaust gas. To reduce CO in the rich exhaust gas, it is sufficient to cause CO to be effectively adsorbed to the CO decreasing catalyst. This can provide exhaust gas having a [H₂/CO] concentration ratio larger than that in conventional exhaust gas purifying catalysts. The CO decreasing catalyst preferably contains compound(s) of noble metal(s) such as Pt, Pd and/or Rh (including any combinations of the noble metals), and alkali metal(s) and/or alkaline earth metal(s) such as Na, K, Rb, Cs, Mg, Ca, Sr and/or Ba (including any combination of the metals). It is preferable that the alkali metal(s) and/or the alkaline earth metal(s) are contained in an amount ranging from 0.1 to 30 g per one liter of the substrate, in the CO decreasing catalyst, from the viewpoints of obtaining a good catalytic performance and lowering a material cost. If the content of the alkali metal(s) and/or the alkaline earth metal(s) is less than 0.1 g per one liter of the substrate, the CO decreasing catalyst cannot exhibit a sufficient CO adsorbing effect so that CO tends to easily pass through the catalyst. If the content exceeds 30 g per one liter of the substrate, not only the material cost is raised but also the noble metal(s) tends to be promoted in deterioration (such as sintering). With the CO decreasing catalyst, it is presumed that the alkali metal and/or alkaline earth metal compound(s) adsorbs or traps CO in the rich exhaust gas and make reaction to convert CO into carbonate, thereby providing exhaust gas having a large [H₂/CO] concentration ratio.

[0051] The CO decreasing catalyst contains the noble metal(s) in number of moles MP and the alkali metal(s) and/or the alkaline earth metal(s) in number of moles MA, in which a ratio MA/MP is preferably within a range of from 0.1 to 10.0, more preferably within a range of from 3.0 to 7.0. With such a ratio MA/MP, a sufficient CO decreasing effect can be obtained. The noble metal(s) and the alkali metal(s) and/or the alkaline earth metals may be carried on a carrier such as alumina powder before the carrier is coated on the substrate such as the honeycomb-type monolithic substrate, or may be carried on the carrier after the carrier is coated on the substrate. If the ratio MA/MP is larger than 10, the CO decreasing ability may be lowered because it is presumed that the catalytic ability of the noble metal(s) is lowered and therefore the CO adsorption performance obtained under the effect of the noble metal(s). Additionally, it may be assumed that the catalytic activity of the noble metal(s) is lowered thereby degrading the performance of the catalyst (as the three-way catalyst). If the ratio MA/PA is smaller than 0.1, a sufficient CO decreasing performance of the catalyst may not be obtained.

[0052] Further, it is preferable that the CO decreasing catalyst additionally possesses the function of a usual three-way catalyst, and therefore may contain cerium (in the form of ceria or the like) and/or zirconia. In this case, exhaust gas from the engine can be effectively purified even in a condition in which the air/fuel ratio of air-fuel mixture to be supplied to the engine is controlled around the stoichiometric value during a high load engine operation such as an acceleration. Although the mechanism of such phenomena presently have not been apparent, it is presumed that coexistence of cerium and/or the like in the catalyst not only suppresses deterioration of the noble metal and lowering of the specific surface area of the catalyst but also provides the effect of suppressing deterioration of the alkali metal(s) and the alkaline earth metal(s).

[0053] The rich exhaust gas CO and H₂ regulating catalyst used in the exhaust gas purifying catalyst of the present invention is one of the shift reaction catalyst, the H₂ low consumption catalyst and the CO decreasing catalyst which function to regulate the concentration of carbon monoxide and hydrogen in the rich exhaust gas, disposed in the exhaust gas passageway of the engine. Otherwise, two or more of the shift reaction catalyst, the H₂ low consumption catalyst and the CO decreasing catalyst may be used in combination as the rich exhaust gas CO and H₂ regulating catalyst. Furthermore, the catalyst metals (components) of the shift reaction catalyst, the H₂ low consumption catalyst and the CO decreasing catalyst may be carried on a single substrate by separately coating the single substrate with slurries which respectively contain the catalyst metals of the above three catalysts. The catalyst metals (components) are preferably carried on porous carrier such as alumina, silica, titania and/or the like. The porous carrier carrying the catalyst metals is preferably supported or carried on the substrate such as a cordierite honeycomb-type monolithic substrate.

[0054] It will be understood that the exhaust gas component regulating device is not limited to the above three kinds of the catalysts and therefore may be a device or means for regulating combustion of the engine. For example, the [H₂/CO] concentration ratio may be regulated by promoting the reaction represented by the following reaction formula (1) upon lowering a combustion temperature of the engine:

CO+H₂O→CO₂+H₂  (1)

[0055] In general, the [H₂/CO] concentration ratio of exhaust gas discharged from the engine is determined according to the thermal equilibrium in the water gas shift reaction represented by the above reaction formula (1), as discussed above. This shift reaction tends to be promoted as the temperature of exhaust gas is low. Accordingly, by lowering the temperature within engine cylinders, the concentration of H₂ can be raised while lowering the concentration of CO in exhaust gas discharged from the engine cylinders. Additionally, by raising the concentration of H₂O in exhaust gas, the shift reaction of the formula (1) can be promoted. For example, by raising the concentration of H₂O in the engine cylinders, the [H₂/CO] concentration ratio can be enlarged.

[0056] As the exhaust gas component concentration regulating device for regulating the concentrations of H₂ and CO in exhaust gas, there are also a NOx adsorbing and reducing catalyst. In other words, the exhaust gas purifying system comprises upstream-side and downstream-side (two) NOx adsorbing and reducing catalysts in the exhaust gas passageway of the engine, disposed in series with each other. With this exhaust gas purifying system, the reaction represented by the reaction formula (1) is promoted similarly in case that the above-mentioned rich exhaust gas CO and H₂ regulating catalyst is used for the exhaust gas component concentration regulating device. As a result, the upstream-side NOx adsorbing and reducing catalyst can supply exhaust gas having a large [H₂/CO] concentration ratio, into the downstream-side NOx adsorbing and reducing catalyst, thereby particularly improving a NOx reduction efficiency in exhaust gas.

[0057] In case that a three-way catalyst is disposed in place of the upstream-side NOx adsorbing and reducing catalyst and that the three-way catalyst is heated, when the air/fuel ratio of exhaust gas is richer (in fuel) than the stoichiometric value or around the stoichiometric value, reducing agents such as CO and H₂ required for releasing and reducing adsorbed NOx are merely consumed. This may lower the NOx reduction performance of the downstream-side NOx adsorbing and reducing catalyst.

[0058] The temperature of the upstream-side NOx adsorbing and reducing catalyst is preferably controlled within a range of from 200 to 500° C. when the air-fuel ratio of exhaust gas is richer than the stoichiometric value or around the stoichiometric value. By this, the adsorbed NOx (nitrogen oxides) can be reduced into N₂ (nitrogen) while exhaust gas discharged from the upstream-side NOx adsorbing and reducing catalyst has such a gas component that the concentrations of carbon monoxide and hydrogen are respectively not more than 2.0% by volume and not less than 0.5% by volume and that the concentration ratio (volume) of [hydrogen (H₂)/carbon monoxide (CO)] is not smaller than 0.5.

[0059] The ratio of [H₂/CO] is preferably not smaller than 1.0 and more preferably not smaller than 10 from the viewpoint of improving the conversion efficiency of NOx into N₂. At the catalyst temperature of lower than 200° C., the H₂ concentration of not less than 0.5% by volume can be obtained while the conversion efficiency of CO is sharply lowered so that the CO concentration of not more than 2.0% by volume cannot be ensured, and therefore the downstream-side NOx adsorbing and reducing catalyst unavoidably undergoes poisoning of CO adsorption so that the NOx removing performance of the catalyst may not be improved. When the catalyst temperature exceeds 500° C., consumption of H₂ increases in the upstream-side NOx adsorbing and reducing catalyst, so that H₂ to be supplied to the downstream-side NOx adsorbing and reducing catalyst may not be ensured.

[0060] The upstream-side NOx adsorbing and reducing catalyst may equipped with a catalyst condition detecting means or device and a temperature control means or device. The catalyst condition detecting device is adapted to detect a condition of the upstream-side NOx adsorbing and reducing catalyst, and the temperature control device is adapted to control the temperature of the catalyst and/or the temperature of exhaust gas to flow into the catalyst in accordance with a detection result of the catalyst condition detecting device, which is effective for improving the NOx reduction performance of the exhaust purifying system for the engine. As shown in FIGS. 6A, 6B and 6C which respectively illustrate the exhaust gas purifying systems according to the present invention, a temperature sensor is used as the catalyst condition detecting device, and an electric heater is used as the temperature control device. The above mentioned “catalyst condition” means the detection result or the like as to whether, for example, temperature, amount and/or the like of exhaust gas flowing into the upstream-side NOx adsorbing and reducing catalyst is within a certain range or not. It will be understood that the above temperature control device is operated in accordance with this detecting result. In each exhaust gas purifying system of FIGS. 6A, 6B and 6C, a signal (representative of the catalyst temperature) from the temperature sensor is supplied to a control unit in which judgment is made as to whether the temperature of the NOx adsorbing and reducing catalyst is within the certain range or not to provide the detection result. The control unit outputs a signal representative of the detection result to an electric source for the heater so that the heater generates heat to heat the NOx adsorbing and reducing catalyst.

[0061] As the above temperature control device, in concrete, a device or means for raising the temperature of the upstream-side NOx adsorbing and reducing catalyst may be operated when the temperature of the upstream-side NOx adsorbing and reducing catalyst is lower than 200° C., whereas a device or means for lowering the temperature of the NOx adsorbing and reducing catalyst may be operated when the temperature of the catalyst exceeds 500° C. Examples of the temperature raising device are a transmission ratio temperature raising device or means, a spark timing temperature raising device or means, a secondary fuel temperature raising device or means, a secondary air temperature raising device or means, the electric heater for heating exhaust gas, the electric heater for heating the catalyst, and the like which are used singly or in combination. The transmission ratio temperature raising device is adapted to increase a transmission ratio of an automatic transmission provided to the internal combustion engine thereby raising the temperature of exhaust gas. The spark timing temperature raising device is adapted to retard the spark timing of the engine relative to that during a normal engine operation thereby to raise the temperature of exhaust gas. The secondary fuel temperature raising device is adapted to supply fuel (as secondary fuel) into the exhaust gas passageway upstream of the upstream-side NOx adsorbing and reducing catalyst thereby raising the temperature of exhaust gas to be supplied to the catalyst. The secondary air temperature raising device is adapted to supply air (as secondary air) into the upstream-side NOx adsorbing and reducing catalyst thereby raising the temperature of the catalyst. The electric heater for heating exhaust gas is adapted to heat exhaust gas to flow into the upstream-side NOx adsorbing and reducing catalyst. The electric heater for heating the catalyst is adapted to heat the upstream-side NOx adsorbing and reducing catalyst. Accordingly, by virtue of the above temperature control devices, the temperature of the upstream-side NOx adsorbing and reducing catalyst can be controlled within a desired range.

[0062] An example of the temperature lowering device is a device for injecting a cooling agent (such as water) into the exhaust gas passageway upstream of the upstream-side NOx adsorbing and reducing catalyst. With this device, the temperature of the catalyst can be easily controlled with a desired range.

[0063] The temperature raising and lowering devices may be used in the exhaust gas purifying system, in which they are suitably selected to be operated in accordance with the temperature of the upstream-side NOx absorbing and reducing catalyst. The temperature control device may be adapted to control also the temperature of the downstream-side NOx adsorbing and reducing catalyst or may be provided to the downstream-side NOx adsorbing and reducing catalyst, which requires increasing the capacity or ability of the temperature control device thereby unavoidably increasing energy consumption. Using the above catalyst condition detecting device in combination with the temperature control device can achieve a temperature control for the upstream-side NOx adsorbing and reducing catalyst with a necessary and minimum energy thereby preventing useless energy consumption.

[0064] The upstream-side NOx adsorbing and reducing catalyst functions to adsorb (trap) NOx during the lean operation of the engine and reduce the adsorbed NOx into N₂ during the rich and stoichiometric operations of the engine. Therefore, the upstream-side NOx adsorbing and reducing catalyst contains the noble metal(s) such as Pt, Pd and/or Rh (including any combinations of the noble metals), and the porous carrier or substrate carrying the noble metals, such as alumina and/or the like. The upstream-side NOx adsorbing and reducing catalyst further contains alkali metals and/or alkaline earth metals such as sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and/or the like (including any combinations of the alkali metals and alkaline earth metals). It is presumed that, by virtue of the alkali metal(s) and/or alkaline earth metal(s), the CO adsorbing ability of the catalyst is improved in the rich exhaust gas, and therefore reaction between adsorbed NOx and adsorbed CO is promoted thereby forming exhaust gas having a large [H₂/CO] ratio.

[0065] It is preferable that the upstream-side NOx adsorbing and reducing catalyst has also a function serving as a usual three-way catalyst, and therefore may contain cerium (Ce), zirconium (Zr), lanthanum (La) and/or the like which are usually employed in a three-way catalyst. It is more preferable that the upstream-side NOx adsorbing and reducing catalyst contains platinum (Pt) and/or rhodium (Rh), and cerium compound (such as ceria). In this regard, it is preferable that the above cerium compound is contained in an amount (calculated as cerium oxide) ranging from 5 to 100 g per one liter of the monolithic substrate. By virtue of this, the reaction between the adsorbed NOx and the adsorbed CO is further promoted in the rich or stoichiometric exhaust gas, thereby making it possible to supply exhaust gas having a high [H₂/CO] ratio to the downstream-side NOx adsorbing and reducing catalyst.

[0066] It is also preferable that the upstream-side NOx adsorbing and reducing catalyst contains Pt and/or Rh, and titanium (Ti) and/or zirconium compound. By this, the reaction between the adsorbed NOx and the adsorbed CO is further improved in the rich or stoichiometric exhaust gas, thereby making it possible to supply exhaust gas having a high [H₂/CO] ratio to the downstream-side NOx adsorbing and reducing catalyst. In this regard, the upstream-side NOx adsorbing and reducing catalyst contains in an amount (calculated as oxide) ranging preferably from 5 to 100 g and more preferably from 10 to 50 g per one liter of the monolithic substrate.

[0067] In case that the NOx adsorbing and reducing catalyst has a function as a three-way catalyst, exhaust gas can be effectively purified in a condition (such as during engine acceleration) where the engine is operated at a high engine load under control of the air-fuel ratio of air-fuel mixture around the stoichiometric value. Composite oxide containing Zr, La and/or the like may be used in place of Zr, La and/or the like. Additionally, Pd may be added to the upstream-side NOx adsorbing and reducing catalyst, which increases a low temperature activity of the catalyst as the three-way catalyst.

[0068] The upstream-side NOx adsorbing and reducing catalyst includes porous carrier on which catalyst components such as noble metal(s) is carried. The porous carrier is formed of the material such as alumina, silica, silica-alumina, titania or the like, though the material is not limited to a particular one. It is preferable that the porous carrier is formed of alumina which is high in heat resistance and in characteristics for dispersing noble metal(s). Additionally, the porous carrier is preferably coated on the honeycomb-type monolithic substrate formed of cordierite or metal. The porous carrier formed of the material such as alumina or silica may be coated on pellet-type substrate. Otherwise, the porous carrier formed of the material such as alumina or silica may be formed into the honeycomb-type or the pellet-type. It will be understood that the material of the porous carrier of the upstream-side NOx adsorbing and reducing catalyst may be the same as or different from that of the downstream-side NOx adsorbing and reducing catalyst.

[0069] Since the upstream-side NOx adsorbing and reducing catalyst is thermally controlled under heating or the like in the exhaust gas purifying system, it is preferable to dispose a plurality of catalysts arranged from the upstream-side to the downstream-side relative to flow of exhaust gas. However, the respective catalysts may be supported on a single substrate so as to be formed into a one-piece catalyst structure, in which only an upstream part (corresponding to the upstream-side NOx adsorbing and reducing catalyst) of the one-piece catalyst structure is thermally controlled. Additionally, the exhaust gas purifying system may take an arrangement in which the three NOx adsorbing and reducing catalysts are disposed in series as shown in FIG. 6B, or may take another arrangement in which a three-way catalyst is disposed on the upstream-side of the two NOx adsorbing and reducing catalysts as shown in FIG. 6C.

[0070] Here, manner of temperature control using the above-mentioned catalyst condition detecting device and temperature control device will be discussed on two (first and second) modes of the exhaust gas purifying systems.

[0071] [First Mode]

[0072] As shown in FIGS. 6A to 6C, the exhaust gas purifying system includes the temperature sensor serving as the catalyst condition detecting device, and the heater, the electric source for the heater and the control unit which serve as the temperature control device. The catalyst condition detecting device and the temperature control device are disposed to heat the upstream-side NOx adsorbing and reducing catalyst.

[0073] The temperature control of the exhaust gas purifying system is executed in accordance with a flowchart shown in FIG. 7A.

[0074] First at step S201, an inlet temperature T0 (or temperature at a position immediately upstream) of the upstream-side NOx adsorbing and reducing catalyst is measured or detected by the temperature sensor. At step S202, judgment is made by the control unit as to whether the inlet temperature T0 is higher than a standard temperature T or not, i.e., whether a relationship T0 >T is established or not. When the relationship T0>T is established, the heater is switched OFF, i.e., the electric source for the heater is switched OFF by the control unit at step S206. When a relationship T0≦T is established, the heater is kept ON, i.e., the electric source for the heater is kept ON by the control unit at step S203. Then, the inlet temperature T0 of the upstream-side NOx adsorbing and reducing catalyst upon the heater being kept ON is detected at S204. The detected inlet temperature T1 is newly set as T0 at step S205. When T0 lowers to reach T after the heater is switched OFF at step S207, detection or measurement of the inlet temperature T0 of the NOx adsorbing and reducing catalyst is again carried out at step S208. FIG. 7B shows a manner of regulating the electric source for the heater in terms of the inlet temperature (° C.) of the upstream-side NOx adsorbing and reducing catalyst in connection with the above temperature control, in which the electric source for the heater is switched ON when the inlet temperature is not higher that 200° C., whereas the electric source is switched OFF when the inlet temperature is higher than 200° C.

[0075] [Second Mode]

[0076] Exhaust gas purifying systems respectively shown in FIGS. 8A and 8B are similar to those in FIGS. 6A and 6C with the exception that a fuel injector and an air injector are used in place of a heating section (including the heater and the electric source) of the exhaust purifying systems of FIGS. 6A and 6C. The fuel injector is adapted to inject fuel (hydrocarbons) to the upstream side of the NOx adsorbing and reducing catalyst under control of the control unit. The air injector is adapted to inject air to the upstream side of the NOx adsorbing and reducing catalyst under control of the control unit.

[0077] The temperature control of the exhaust gas purifying system is executed in accordance with a flowchart shown in FIG. 9. A manner of the temperature control in FIG. 9 is similar to that in FIG. 7A except for the following steps: When the relationship T0>T is established upon result of the judgment at step S402, control of the fuel and air injectors are switched OFF to stop fuel and air injections at step S406. When a relationship T0≦T is established, the control of the fuel and air injectors are kept ON to make fuel and air injections at step S403. Then, the inlet temperature T0 of the upstream-side NOx adsorbing and reducing catalyst upon the fuel and air injections is detected at S404. It will be understood that steps 401, 402 404, 405, 407 and 408 correspond respectively to steps 201, 202, 204, 405, 407 and 208 in the flowchart of FIG. 7A.

[0078] A second embodiment of the exhaust gas purifying system according to the present invention will be discussed hereinafter with reference to FIG. 2.

[0079] The exhaust gas purifying system is for an internal combustion engine and comprises a NOx adsorbing (trapping) catalyst disposed in an exhaust gas passageway of the engine, the NOx adsorbing catalyst functioning to adsorb and release nitrogen oxides in accordance with a temperature of exhaust gas. The NOx adsorbing catalyst functions to adsorb nitrogen oxides in exhaust gas in a first exhaust gas temperature range in which exhaust gas has a temperature ranging from a level at engine starting to 140° C. and release the adsorbed nitrogen oxides in a second exhaust gas temperature condition in which exhaust gas has a temperature of not lower than 200° C. Exhaust gas discharged from the NOx adsorbing catalyst has a concentration ratio (volume) of [nitrogen oxides (NOx)/carbon monoxide (CO)] which is not larger than 0.3 in the first exhaust gas temperature range. Additionally, an oxidizing catalyst is disposed in the exhaust gas passageway downstream of the NOx adsorbing catalyst, for oxidizing oxidizable components in exhaust gas discharged from the NOx adsorbing catalyst. A major part of the exhaust gas passageway is formed in the exhaust pipe.

[0080] Thus, in this exhaust gas purifying system, the NOx adsorbing catalyst for regulating adsorption (trap) and release of NOx in accordance with the temperature of exhaust gas is disposed in the exhaust gas passageway at the upstream side, while the oxidizing catalyst is disposed in the exhaust gas passageway downstream of the NOx adsorbing catalyst. The NOx adsorbing catalyst causes exhaust gas having a small [nitrogen oxides (NOx)/carbon monoxide (CO)] concentration ratio to flow to the oxidizing catalyst when exhaust gas is in a low temperature region, thereby effectively removing or oxidizing HC, CO and NOx in the rich exhaust gas. In other words, the NOx adsorbing catalyst functions to adsorb nitrogen oxides in exhaust gas in the first exhaust gas temperature range in which exhaust gas has a temperature ranging from a level at engine starting to 140° C. and release the adsorbed nitrogen oxides in the second exhaust gas temperature range in which exhaust gas has a temperature of not lower than 200° C. Additionally, in the first exhaust gas temperature range, the [nitrogen oxides/carbon monoxide] concentration ratio is regulated to be not larger than 0.3. The exhaust gas purifying system of this embodiment is, for example, shown in FIG. 11.

[0081] The NOx adsorbing catalyst and the oxidizing catalyst includes porous carrier on which catalyst components such as noble metal(s) is carried. The porous carrier is formed of a material such as alumina, silica, silica-alumina, titania or the like, though the material is not limited to a particular one. It is preferable that the porous carrier is formed of alumina which is high in heat resistance and in characteristics for dispersing noble metal(s). Additionally, the porous carrier is preferably coated on a honeycomb-type monolithic substrate formed of cordierite or metal. The porous carrier formed of the material such as alumina or silica may be coated on pellet-type substrate. Otherwise, the porous carrier formed of the material such as alumina or silica may be formed into the honeycomb-type or the pellet-type. It will be understood that the material of the porous carrier of the NOx adsorbing catalyst may, be the same as or different from that of the oxidizing catalyst.

[0082] The NOx adsorbing catalyst contains catalyst component(s) or NOx adsorbing material which functions to adsorb (trap) and release NOx by changing the temperature of exhaust gas to be flown to the NOx adsorbing catalyst. It is preferable that the NOx adsorbing catalyst contains Pt (as the catalyst component) in an amount ranging from 0.1 to 10 g per one liter of the substrate such as the honeycomb-type monolithic substrate. The NOx adsorbing catalyst containing Pt adsorbs or traps NOx in the low exhaust gas temperature region, and release the adsorbed NOx when the temperature of exhaust gas is raised. If the amount of Pt is less than 0.1 g per one liter of the substrate, a practically sufficient catalytic activity for NOx cannot be obtained. If the amount of Pt exceeds 10 g per one liter of the substrate, the catalytic activity for NOx cannot be improved corresponding to an increased amount of Pt, so as to be ineffective for improving the catalytic activity. It will be understood that Pt may be carried on the above-mentioned porous carrier by coating the porous carrier with Pt compound(s) such as chloride, nitrate or the like of Pt under conventional methods such as an impregnation method, a spray method, or a slurry mixing method. In the impregnation method, the porous carrier is impregnated with a fluid containing Pt compound(s). In the spray method, a fluid containing Pt compound(s) is sprayed onto the porous carrier. In the slurry mixing method, the porous carrier is mixed in a fluid containing Pt compound(s) to form a slurry.

[0083] It is preferable that the NOx adsorbing catalyst further contains, as the NOx adsorbing agent or catalyst component, alkali metal(s), alkaline-earth metal(s) and/or rare earth element(s) preferably in an amount ranging from 1 to 10 g per one liter of the substrate. Examples of alkali metal (elements of the group 2A in the periodic table of elements) are lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs) and francium (Fr). Examples of alkaline-earth metal are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr) and barium (Ba). Examples of rare earth element are scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr) and neodymium (Nd). The NOx adsorbing catalyst preferably contains the above alkali metal(s), alkaline-earth metal(s) and/or rare earth element(s) serving as the NOx adsorbing material, preferably in an amount ranging from 1 to 50 g per one liter of the substrate (such as the honeycomb-type monolithic substrate). If the amount is less than 1 g per one liter of the substrate, a NOx adsorbing (trapping) ability of the NOx adsorbing catalyst is low thereby providing an insufficient NOx reducing or removing performance. If the amount exceeds 50 g per one liter of the substrate, a temperature at which NOx can be released is raised so that NOx may not sufficiently release; and an oxidizing ability of the catalyst is lowered so as to cause disadvantages such as impeding oxidation of NO to nitrogen dioxide (NO₂).

[0084] It is also preferable that the NOx adsorbing catalyst functions to adsorb NOx in exhaust gas at a temperature ranging from 100 to 140° C. so as to lower the concentration of NOx in exhaust gas. The NOx adsorbing catalyst containing, as the NOx adsorbing material, Pt and the above-mentioned alkali metal(s) or/and the like can adsorb NOx when the temperature of exhaust gas is within a range of from 100 to 140° C. thereby causing exhaust gas having a small [NOx/CO] concentration ratio to flow into the oxidizing catalyst located downstream of the NOx adsorbing catalyst, thus preventing the oxidizing catalyst from being lowered in activity for oxidizing CO. When the temperature of exhaust gas rises, the adsorbed NOx is released so as to accomplish regeneration of the NOx adsorbing catalyst.

[0085] The oxidizing catalyst preferably contains catalyst components for improving the activity for oxidizing CO at a low temperature, exhibiting a high activity for oxidizing HC and so-called soluble organic fraction (SOF) in lean exhaust gas which has an air/fuel ratio leaner (larger) than a stoichiometric value, particularly from a time immediately after engine starting.

[0086] The oxidizing catalyst preferably contains Pt in an amount ranging from 0.5 to 20 g per one liter of the substrate (such as the honeycomb-type monolithic substrate) thereby effectively oxidizing or removing CO, HC and SOF. If the amount of Pt is less than 0.5 g per one liter of the substrate, a practically sufficient catalytic activity cannot be obtained. If the amount of Pt exceeds 20 g per one liter of the substrate, the catalytic activity cannot be improved corresponding to an increased amount so that an effective use of Pt cannot be made. In the oxidizing catalyst, Pt is carried on the above-mentioned porous carrier by coating the porous carrier with Pt compound(s) such as chloride, nitrate or the like of Pt under conventional methods such as the impregnation method, the spray method, or the slurry mixing method. The porous carrier carrying Pt is preferably coated on the substrate such as the honeycomb-type monolithic substrate.

[0087] It is also preferable that the oxidizing catalyst contains zeolite in an amount ranging from 10 to 100 g per one liter of the substrate such as the honeycomb-type monolithic substrate.

[0088] Examples of zeolite are mordenite, MFI, β-zeolite and the like, in which they are used singly or in combination. If the amount of zeolite is less than 10 g per one liter of the substrate, an absorption amount of HC, SOF and particulate matter (PM) is insufficient. If the amount of zeolite exceeds 100 g per one liter of the substrate, there is the possibility of the pores of the porous carrier being clogged with PM. By virtue of zeolite contained in the oxidizing catalyst, HC can be adsorbed in zeolite even when the temperature of exhaust gas is not higher than 140° C. thereby suppressing poisoning with HC. This prevents an oxidation catalytic activity of Pt from lowering thus improving a low temperature catalytic activity for CO. It is also preferable that zeolite is mixed with the porous carrier to form a layer coated on the substrate. This improves an effect for suppressing the HC poisoning to Pt as compared with a method in which zeolite is coated on a layer of the porous carrier carrying Pt.

[0089] Additionally, it is preferable that the oxidizing catalyst does not contain the above-mentioned alkali metal, alkaline-earth metal and/or rate earth element which serve as the NOx adsorbing material. By this, the oxidizing catalyst cannot adsorb NOx and therefore exhibits a high oxidation catalytic activity when exhaust gas is in a lean region (in which air/fuel ratio is larger or leaner than the stoichiometric level), thereby oxidizing and removing HC and CO under the action of NOx. In this connection, conventional three-way catalysts contain the above-mentioned alkali metal(s) and/or the like, and therefore cannot exhibit a high oxidation catalytic activity when exhaust gas is the lean region. In other words, the conventional three-way catalysts are raised in exhaust gas purifying ability by adding alkali metal(s), rare earth element(s) and/or the like to noble metal(s), in which CO, HC and NOx are simultaneously oxidized and reduced to be removed under catalytic reaction when exhaust gas is in a stoichiometric region (in which air/fuel ratio is generally stoichiometric). It is to be noted that the oxidizing catalyst of the present invention is expected to be high in oxidation catalytic activity in the lean region of exhaust gas, and therefore quite different from the conventional three-way catalysts.

[0090] In operation of the above-discussed exhaust gas purifying system, when the temperature of exhaust gas is within the range of from the level at engine starting to 140° C., NOx is adsorbed or trapped on Pt in the NOx adsorbing catalyst. In case that the NOx adsorbing catalyst contains also alkali metal(s), alkaline-earth metal and/or rare earth element(s), NOx is adsorbed also on them. Under such NOx adsorption, the [NOx/CO] concentration ratio of exhaust gas in the exhaust gas passageway between the NOx adsorbing catalyst and the oxidizing catalyst becomes not larger than 0.3. This exhaust gas having the small [NOx/CO] concentration ratio is flown into the oxidizing catalyst, upon which CO is oxidized to be removed under the oxidation catalytic activity of Pt while HC and SOF are adsorbed on zeolite (in case of using zeolite).

[0091] When the temperature of exhaust gas exceeds 140° C. and reaches a level lower than 200° C., the NOx adsorbing catalyst functions similarly to the above-discussed as follows: NOx is adsorbed or trapped on Pt. In case that the NOx adsorbing catalyst contains also alkali metal(s) and/or the like, NOx is adsorbed also on them. An adsorption equilibrium state of NOx may be maintained.

[0092] At this time, the oxidizing catalyst functions as follows: CO is oxidized to be removed under the oxidation catalytic activity of Pt while HC and SOF are adsorbed on zeolite (in case of using zeolite) or maintained at an adsorption equilibrium state. Additionally, HC and SOF may be released and oxidized under the oxidation catalytic activity of Pt according to a flow rate of exhaust gas.

[0093] When the temperature of exhaust gas reaches a level over 200° C., the absorbed NOx is released from the NOx adsorbing catalyst, thereby accomplishing regeneration of the NOx adsorbing catalyst. In the oxidizing catalyst, CO is oxidized to be removed under the oxidation catalytic activity of Pt while HC and SOF are adsorbed on zeolite so as to be at the adsorption equilibrium state or released to be oxidized under the oxidation catalytic activity of Pt.

[0094] While the exhaust gas purifying systems of the embodiments have been shown and described as including the single NOx adsorbing catalyst and the single oxidizing catalyst, it will be understood that a plurality of the NOx adsorbing catalysts and a plurality of the oxidizing catalysts may be used respectively in place of the single NOx adsorbing catalyst and the single oxidizing catalyst so as to constitute a multiple stage NOx adsorbing catalyst and a multiple stage oxidizing catalyst. Additionally, the NOx adsorbing catalyst and the oxidizing catalyst may be combined to form a single unit. Further, a layer of the catalyst component(s) of the NOx adsorbing catalyst and a layer of the catalyst component(s) of the oxidizing catalyst may be coated on a single substrate such as the honeycomb-type monolithic substrate.

[0095] The exhaust gas purifying systems of the first and second embodiments may be combined to constitute an exhaust gas purifying system. Specifically, the exhaust gas component regulating device and/or the NOx adsorbing and reducing catalyst of the exhaust gas purifying system of the first embodiment and the NOx adsorbing catalyst and/or the oxidizing catalyst of the exhaust gas purifying system of the second embodiment may be combined to constitute an exhaust gas purifying system as shown in FIGS. 3 to 5. The exhaust gas purifying system of FIG. 5 corresponds to that of FIG. 12 which is illustrated in concrete as compared with FIG. 5. In the exhaust gas purifying system of FIG. 5, the NOx adsorbing and reducing catalyst and the NOx adsorbing catalyst may be replaced (in location) with each other so that the former catalyst is located to be downstream of the latter catalyst. It will be understood that the exhaust gas component regulating device, the NOx adsorbing and reducing catalyst and the oxidizing catalyst may be disposed in the exhaust gas passageway in the order mentioned in a direction of from the upstream side to the downstream side relative to flow of exhaust gas so as to constitute an exhaust gas purifying system as shown in FIG. 3. Additionally, the exhaust gas purifying system of the first embodiment may be modified such that the NOx adsorbing and reducing catalyst is replaced with the NOx adsorbing catalyst of the exhaust gas purifying system of the second embodiment. These combination arrangements can simultaneously achieve improvements both in NOx reduction efficiency in lean exhaust gas and in oxidation catalytic activity in exhaust gas in a low temperature region, thus purifying exhaust gas purification at a high efficiency.

EXAMPLES

[0096] The present invention will be more readily understood with reference to the following Examples in comparison with Comparative Examples; however, these Examples are intended to illustrate the invention and are not to be construed to limit the scope of the invention.

Example 1

[0097] Activated alumina powder was impregnated with a solution of dinitrodiammine Pt, and then dried and calcined in air at 400° C. for 1 hour thereby obtaining Pt-carrying alumina powder (Powder A) having a Pt concentration of 3.0% by weight.

[0098] Activated alumina powder was impregnated with an aqueous solution of Rh nitrate, and then dried and calcined in air at 400° C. for 1 hour, thereby obtaining Rh-carrying alumina powder (Powder B) having a Rh concentration of 2.0% by weight.

[0099] A porcelain ball mill was charged with 576 g of Powder A, 86 g of Powder B, 238 g of activated alumina powder and 900 g of water. Mixing and pulverization were made in the porcelain ball mill thereby obtaining a slurry. This slurry was coated on a cordierite honeycomb-type monolithic substrate having a volume of 1.7 liters and 400 cells per square inch. The cells were formed extending throughout the length of the monolithic substrate. The coated monolithic substrate was blown with air to remove excessive slurry in the cells under the action of air stream. Thereafter, the coated monolithic substrate was dried at 130° C. and then calcined in air at 400° C. for 1 hour thereby obtaining a catalyst formed with a coat layer having a weight of 200 g per one liter of the monolithic substrate.

[0100] The thus obtained catalyst was impregnated with an aqueous solution of Ba acetate, and dried and calcined in air at 400° C. for 1 hour thereby obtaining a catalyst (Catalyst A) formed with a coat layer having a weight of 250 g per one liter of the monolithic substrate.

[0101] Activated alumina powder was impregnated with a solution of cerium nitrate, and then dried in air at 600° C. for 1 hour thereby obtaining cerium-carrying alumina powder (Powder C). Powder C was impregnated with a solution of dinitrodiammine Pt, and then dried and calcined in air at 400° C. for 1 hour thereby obtaining Pt and Ce-carrying alumina powder (Powder D) having a cerium concentration of 5.0% by weight and a Pt concentration of 3.0% by weight.

[0102] Powder C was impregnated with an aqueous solution of Rh nitrate, and then dried and calcined in air at 400° C. for 1 hour thereby obtaining Rh and Ce-carrying alumina powder (Powder E) having a cerium concentration of 5.0% by weight and a Rh concentration of 2.0% by weight.

[0103] A porcelain ball mill was charged with 576 g of Powder D, 86 g of Powder E, 238 g of activated alumina powder and 900 g of water. Mixing and pulverization were made in the porcelain ball mill thereby obtaining a slurry. This slurry was coated on a cordierite honeycomb-type monolithic substrate having a volume of 1.3 liters and 400 cells per square inch. The cells were formed extending throughout the length of the monolithic substrate. The coated monolithic substrate was blown with air to remove excessive slurry in the cells under the action of air stream. Thereafter, the coated monolithic substrate was dried at 130° C. and then calcined in air at 400° C. for 1 hour thereby obtaining a catalyst (Catalyst B) formed with a coat layer having a weight of 200 g per one liter of the monolithic substrate.

[0104] An exhaust gas purifying catalyst of Example 1 was constituted by disposing Catalyst B in an exhaust gas passageway of an automotive internal combustion engine, and by disposing Catalyst A in the exhaust gas passageway downstream of Catalyst B. Catalyst B corresponded to the above-mentioned shift reaction catalyst.

Comparative Example 1

[0105] A catalyst C of Comparative Example 1 was produced by repeating the procedure of producing Catalyst B in Example 1 with the exception that activated alumina powder was used in place of Powder C (cerium-carrying alumina powder) in preparation of Catalyst B.

[0106] An exhaust gas purifying system of Comparative Example 1 was constituted by disposing Catalyst C in an exhaust gas passageway of an automotive internal combustion engine, and by disposing Catalyst A in the exhaust gas passageway downstream of Catalyst C.

Example 2

[0107] Catalyst D of Example 2 was produced by repeating the procedure of producing Catalyst B in Example 1 with the exception that titania sol was used in place of the solution of cerium nitrate in preparation of Catalyst B.

[0108] An exhaust gas purifying catalyst of Example 2 was constituted by disposing Catalyst D in an exhaust gas passageway of an automotive internal combustion engine, and by disposing Catalyst A in the exhaust gas passageway downstream of Catalyst D. Catalyst D corresponded to the above-mentioned H₂ low consumption catalyst.

Example 3

[0109] Catalyst E of Example 3 was produced by repeating the procedure of producing Catalyst B in Example 1 with the exception that a solution of zirconium nitrate was used in place of the solution of cerium nitrate in preparation of Catalyst B.

[0110] An exhaust gas purifying catalyst of Example 3 was constituted by disposing Catalyst E in an exhaust gas passageway of an automotive internal combustion engine, and by disposing Catalyst A in the exhaust gas passageway downstream of Catalyst E. Catalyst E corresponded to the above-mentioned H₂ low consumption catalyst.

Example 4

[0111] Catalyst F of Example 4 was produced by repeating the procedure of producing Catalyst B in Example 1 with the exception that a solution of magnesium acetate was used in place of the solution of cerium nitrate in preparation of Catalyst B.

[0112] An exhaust gas purifying catalyst of Example 4 was constituted by disposing Catalyst F in an exhaust gas passageway of an automotive internal combustion engine, and by disposing Catalyst A in the exhaust gas passageway downstream of Catalyst F. Catalyst F corresponded to the above-mentioned CO decreasing catalyst.

Evaluation of Performance of Exhaust Gas Purifying System (1)

[0113] An internal combustion engine having a displacement of 2000 cc was provided with an exhaust system to which the exhaust gas purifying catalyst of Examples and Comparative Example was incorporated. An evaluation test of the exhaust gas purifying system was conducted as follows:

[0114] This engine was operated repeating a changing operational cycle (evaluation mode) including a first engine operation step of 30 seconds on air-fuel mixture having an air/fuel ratio of 20, a second engine operation step of 4 seconds on air-fuel mixture having an air/fuel ratio of 11.0, and a third engine operation step of 5 seconds on air-fuel mixture having an air/fuel ratio of 14.7. The first, second and third engine operation steps were successively made in the order named. Such engine operation was carried out in two manners one of which was to control the temperature of exhaust gas immediately upstream of the exhaust gas purifying system at 200° C. and the other was to control the temperature of exhaust gas immediately upstream of the exhaust gas purifying system at 350° C. During one changing operational cycle in each manner of the engine operation, a concentration (ppm) of an exhaust gas component (HC, CO, NOx) was measured at positions of the exhaust gas passageway upstream and downstream of the exhaust gas purifying system. Such measurement was conducted on the exhaust gas purifying catalyst in a state after a durability test, thereby calculating a conversion rate (%) shown in Table 1. The conversion rate (%) was calculated by [(the concentration of the gas component in the exhaust gas passageway upstream of the exhaust gas purifying system—the concentration of the gas component in the exhaust gas passageway downstream of the exhaust gas purifying system/the concentration of the gas component in the exhaust gas passageway upstream of the exhaust gas purifying system)×100].

[0115] The durability test was conducted as follows: The exhaust gas purifying catalyst of Examples and Comparative Examples was provided in an exhaust system of an internal combustion engine having a displacement of 4400 cc. The engine was operated for 50 hours in which the temperature of exhaust gas immediately upstream of the exhaust gas purifying system was kept at 750° C.

[0116] The results of the evaluation test on the exhaust gas purifying systems of Examples 1 to 4 and Comparative Example 1 are shown in Table 1. TABLE 1 Conversion rate (%) 200° C. 350° C. HC CO NOx HC CO NOx Example 1 90 95 85 92 96 90 Comp. 90 95 70 92 96 80 Example 1 Example 2 89 95 85 92 96 90 Example 3 92 95 88 92 96 90 Example 4 92 95 90 92 96 92

Example 5

[0117] Activated alumina powder was impregnated with a solution of dinitrodiammine Pt, and then dried and calcined in air at 400° C. for 1 hour thereby obtaining Pt-carrying alumina powder (Powder F) having a Pt concentration of 2.0% by weight.

[0118] A porcelain ball mill was charged with 750 parts by weight of Powder F, 1250 parts by weight of nitric acid-acidic alumina sol (having a solid content of 20% by weight) and 500 parts by weight of pure water. Mixing and pulverization were made for 1 hour in the porcelain ball mill thereby obtaining a slurry.

[0119] This slurry was coated on a cordierite honeycomb-type monolithic substrate having a volume of 0.3 liter and 400 cells per square inch, in which the cells were formed extending throughout the length of the monolithic substrate. The monolithic substrate had a thickness of walls (defining each cell) of 6 mil, so that the walls of the cells were coated with the slurry. The coated monolithic substrate was blown with air to remove excessive slurry in the cells under the action of air stream. Thereafter, the coated monolithic substrate was dried at 130° C. and then calcined at 500° C. for 1 hour thereby obtaining a catalyst (Catalyst G) formed with a coat layer having a weight of 200 g per one liter of the monolithic substrate.

[0120] A porcelain ball mill was charged with 400 parts by weight of Powder F, 350 parts by weight of mordenite, 1250 parts by weight of nitric acid-acidic alumina sol (having a solid content of 20% by weight) and 500 parts by weight of pure water. Mixing and pulverization were made for 1 hour thereby obtaining a slurry. This slurry was coated on a cordierite honeycomb-type monolithic substrate having a volume of 0.5 liter and 400 cells per square inch, in which the cells were formed extending throughout the length of the monolithic substrate. The monolithic substrate had a thickness of walls (defining each cell) of 6 mil, so that the walls of the cells were coated with the slurry. The coated monolithic substrate was blown with air to remove excessive slurry in the cells under the action of air stream. Thereafter, the coated monolithic substrate was dried at 130° C. and then calcined at 500° C. for 1 hour thereby obtaining a catalyst (Catalyst H) formed with a coat layer having a weight of 200 g per one liter of the monolithic substrate.

[0121] An exhaust gas purifying catalyst of Example 5 was constituted by disposing Catalyst G in a gas passageway, and by disposing Catalyst H in the gas passageway downstream of Catalyst G. The Catalysts G, H were located adjacent each other, forming a clearance of 1 to 10 cm between them.

Example 6

[0122] Catalyst I of Example 6 was produced by repeating the procedure of producing Catalyst G in Example 5 except for the following: Catalyst G was dipped in an aqueous solution of barium acetate, and then drawn up from the aqueous solution and dried at 250° C. The thus dried catalyst was calcined at 300° C. for 1 hour, thereby obtaining a catalyst (Catalyst I) carrying Ba.

[0123] An exhaust gas purifying system of Example 6 was constituted by disposing Catalyst I in a gas passageway of an automotive internal combustion engine, and by disposing Catalyst H in the gas passageway downstream of Catalyst I. The Catalysts I, H were located adjacent each other, forming the clearance therebetween.

Comparative Example 3

[0124] Activated alumina powder was impregnated with an aqueous solution of palladium chloride, and then dried and calcined in air at 400° C. for 1 hour thereby obtaining Pd-carrying alumina powder (Powder G) having a Pd concentration of 2.0% by weight.

[0125] A porcelain ball mill was charged with 750 parts by weight of Powder G, 1250 parts by weight of nitric acid-acidic alumina sol (having a solid content of 20% by weight) and 500 parts by weight of pure water. Mixing and pulverization were made for 1 hour in the ball mill, thereby obtaining a slurry.

[0126] This slurry was coated on a cordierite honeycomb-type monolithic substrate having a volume of 0.5 liter and 400 cells per square inch, in which the cells were formed extending throughout the length of the monolithic substrate. The monolithic substrate had a thickness of walls (defining each cell) of 6 mil, so that the walls of the cells were coated with the slurry. The coated monolithic substrate was blown with air to remove excessive slurry in the cells under the action of air stream. Thereafter, the coated monolithic substrate was dried at 130° C. and then calcined at 500° C. for 1 hour thereby obtaining a catalyst (Catalyst J) formed with a coat layer having a weight of 200 g per one liter of the monolithic substrate.

[0127] An exhaust gas purifying system of Comparative Example 3 was constituted by disposing Catalyst G in a gas passageway of an automotive internal combustion engine, and by disposing Catalyst J in the gas passageway downstream of Catalyst G. The Catalysts G and J were located adjacent each other, forming the clearance between them.

Comparative Example4

[0128] A porcelain ball mill was charged with 750 parts by weight of activated alumina, 1250 parts by weight of nitric acid-acidic alumina sol (having a solid content of 20% by weight) and 500 parts by weight of pure water. Mixing and pulverization were made for 1 hour in the ball mill.

[0129] This slurry was coated on a cordierite honeycomb-type monolithic substrate having a volume of 0.3 liter and 400 cells per square inch, in which the cells were formed extending throughout the length of the monolithic substrate. The monolithic substrate had a thickness of walls (defining each cell) of 6 mil, so that the walls of the cells were coated with the slurry. The coated monolithic substrate was blown with air to remove excessive slurry in the cells under the action of air stream. Thereafter, the coated monolithic substrate was dried at 130° C. and then calcined at 500° C. for 1 hour thereby obtaining a catalyst formed with a coat layer having a weight of 250 g per one liter of the monolithic substrate.

[0130] Subsequently, the thus obtained catalyst was dipped in an aqueous solution (having a certain concentration) of dinitrodiammine Pt and then drawn up from the aqueous solution. Thereafter, the catalyst was blown to remove excessive water content, and then dried at 250° C. so as to obtain a catalyst carrying Pt.

[0131] Next, the thus obtained catalyst was dipped in an aqueous solution (having a certain concentration) of rhodium nitrate and then drawn up from the aqueous solution. Thereafter, the catalyst was blown to remove excessive water content, and then dried at 250° C. thereby obtaining a Pt and Rh-carrying catalyst having a Pt concentration of 2.73% by weight and a Rh concentration of 0.27% by weight.

[0132] Further, the thus obtained Pt and Rh-carrying catalyst was dipped in a solution (having a certain concentration) of barium acetate and then drawn up from the solution. Thereafter, the catalyst was blown to remove excessive water content, and then dried at 250° C. and calcined at 300° C. for 1 hour, thereby obtaining a catalyst (Catalyst K) carrying Ba in an amount (calculated as BaO) of 10 g per one liter of the monolithic substrate.

[0133] An exhaust gas purifying system of Comparative Example 4 was constituted by disposing Catalyst G in a gas passageway of an automotive internal combustion engine, and by disposing Catalyst K in the gas passageway downstream of Catalyst G. The Catalysts G, K were located adjacent each other, forming the clearance between them.

[0134] Catalyst components of the exhaust gas purifying systems of Examples 5 and 6 and Comparative Examples 2 to 4 are summarized in Table 2. TABLE 2 NO/CO Upstream- Downstream- downstream of Conversion side side NOx adsorbing rate(%) catalyst catalyst catalyst CO HC Example 5 Pt/alumina Pt + zeolite 0.2 86 61 Example 6 Pt/alumina + Pt + zeolite 0.15 87 60 Ba Comp. — Pt + zeolite 1.0 15 60 Example 2 Comp. Pt/alumina Pd + zeolite 0.2 52 63 Example 3 Comp. Pt/alumina Pt, Rh 0.2 40 0 Example 4

Evaluation of Performance of Exhaust Gas Purifying System (2)

[0135] <Evaluation A>

[0136] Tests for evaluating catalytic activity were conducted on the exhaust gas purifying systems of Examples 5 and 6 and Comparative Examples 2 to 4. The test for each exhaust gas purifying system was carried out in test conditions shown at a part of “Evaluation test” in Table 3. In this test, the upstream-side catalyst (NOx adsorbing catalyst) and the downstream-side catalyst (oxidizing catalyst) were located adjacent and axially aligned with each other, and disposed in a casing which forms part of the gas passageway. A model gas for (performance) evaluation purpose simulated exhaust gas of an automotive vehicle and was flown through the gas passageway at a flow rate of 60 liters per minute, in which the temperature of the gas in the gas passage immediately upstream (at the inlet) of the exhaust gas purifying system was kept at 130° C. The model gas had a composition shown in the part of “Evaluation test” in Table 3. In these tests, the upstream-side (NOx adsorbing) catalyst had a volume of 0.02 liter, while the downstream-side (oxidizing) catalyst had a volume of 0.04 liter. In these tests, a concentration (ppm) of a gas component (CO, HC) was measured at positions of the gas passageway upstream and downstream of the exhaust gas purifying system. Such measurement was made on the exhaust gas purifying system in a state after completion of a durability test, thereby calculating a conversion rate (%) which is shown in Table 2. The conversion rate (%) was calculated by [(the concentration of the gas component in the gas passageway upstream of the exhaust gas purifying system—the concentration of the gas component in the gas passageway downstream of the exhaust gas purifying system/the concentration of the gas component in gas passageway upstream of the exhaust gas purifying system)×100].

[0137] The durability test was conducted on each of the exhaust gas purifying systems of Examples 5 and 6 and Comparative Examples 2 to 4 in test conditions shown at a part of “Durability test” in Table 3. In this test, the upstream-side catalyst (NOx adsorbing catalyst) and the downstream-side catalyst (oxidizing catalyst) were located adjacent and axially aligned with each other, and disposed in a casing which forms part of a gas passageway. A model gas for durability test purpose simulated exhaust gas of an automotive vehicle and was flown through the gas passageway and through the upstream-side and downstream-side catalysts at a flow rate of 60 liters per minute for 10 hours, in which the temperature of the model gas in the gas passage immediately upstream (at the inlet) of the exhaust gas purifying system was kept at 500° C. The model gas had a composition shown in the part of “Durability test” in Table 3. TABLE 3 Durability test Temp. of model gas at inlet 500° C. Test time 10 hours Composition of model gas CO 500 ppm at inlet of exhaust gas purifying HC(C₃H₆) 300 ppm system NO 500 ppm O₂  10 vol. % SO₂  20 ppm CO₂  10 vol. % H₂O  10 vol. % N₂ balance Evaluation test Temp. of model gas at inlet 130° C. Composition of model gas CO 1000 ppm at inlet of exhaust gas purifying HC(C₃H₆)  300 ppm system NO  500 ppm O₂  10 vol. % CO₂  10 vol. % H₂O  10 vol. % N₂ balance

[0138] <Evaluation B>

[0139] The same tests as those of <Evaluation A> for evaluating catalytic activity were conducted on a variety of modes of the exhaust gas purifying system of Example 5, different in volume of the upstream-side (NOx adsorbing) catalyst and in volume of the downstream-side (oxidizing) catalyst as shown in Table 4. The exhaust gas purifying systems of Example 5 had been subjected to the durability test in <Evaluation B>. In these tests, the conversion rates for CO and HC were measured to evaluate the catalytic activities of the exhaust gas purifying systems. The results of these tests are shown in Table 4. TABLE 4 NO/CO downstream of NOx Volume (liter) of Volume (liter) of adsorbing upstream-side downstream-side Conversion rate (%) catalyst catalyst catalyst CO HC 0.1 0.03 0.04 90 62 0.2 0.02 0.04 86 61 0.3 0.01 0.04 80 65 0.4 0.005 0.04 40 63 0.5 None 0.04 15 60

[0140] The test results of <Evaluation A> and <Evaluation B> reveal that the exhaust gas purifying systems of Examples 5 and 6 are high in catalytic activity and exhibit high conversion performances for HC and CO even in a low exhaust gas temperature region while providing high conversion performances for HC and CO, as compared with the exhaust gas purifying systems of Comparative Examples 2 to 4.

Example 7

[0141] Catalyst L of Example 7 was produced by repeating the procedure of producing Catalyst B of Example 1 with the exception that Catalyst B was additionally impregnated with an aqueous solution of Ba acetate, and dried and calcined in air at 400° C. for 1 hour. Catalyst L was formed with a coat layer having a weight of 250 g per one liter of the monolithic substrate.

[0142] An exhaust gas purifying system of Example 7 was constituted by disposing Catalyst L in an exhaust gas passageway of an automotive internal combustion engine, and by disposing Catalyst A in the exhaust gas passageway downstream of Catalyst L. Catalyst L corresponded to the above-mentioned NOx adsorbing and reducing catalyst.

Comparative Example 5

[0143] An exhaust gas purifying system of Comparative Example was constituted by repeating the procedure of Example 7; however, the temperature (or the inlet temperature) of exhaust gas immediately upstream of the upstream-side catalyst in the exhaust gas purifying system was set at 300° C. in an evaluation test discussed after.

Comparative Example 6

[0144] Catalyst M of Comparative Example 6 was produced by repeating the procedure of producing Catalyst L of Example 7 with the exception that Catalyst B was not impregnated with an aqueous solution of Ba acetate. Catalyst M was a three-way catalyst.

[0145] An exhaust gas purifying system of Comparative Example 6 was constituted by disposing Catalyst M in an exhaust gas passageway of an automotive internal combustion engine, and by disposing Catalyst A in the exhaust gas passageway downstream of Catalyst M. Catalyst M corresponded to the above-mentioned NOx adsorbing and reducing catalyst.

Example 8

[0146] Catalyst N of Example 8 was produced by repeating the procedure of producing Catalyst L of Example 7 with the exception that titania sol was used in place of the solution of cerium nitrate in the process of producing Catalyst L.

[0147] An exhaust gas purifying system of Example 8 was constituted by disposing Catalyst N in an exhaust gas passageway of an automotive internal combustion engine, and by disposing Catalyst A in the exhaust gas passageway downstream of Catalyst N. Catalyst N corresponded to the above-mentioned NOx adsorbing and reducing catalyst.

Example 9

[0148] Catalyst O of Example 9 was produced by repeating the procedure of producing Catalyst L of Example 7 with the exception that a solution of zirconium nitrate was used in place of the solution of cerium nitrate in the process of producing Catalyst L.

[0149] An exhaust gas purifying system of Example 9 was constituted by disposing Catalyst O in an exhaust gas passageway of an automotive internal combustion engine, and by disposing Catalyst A in the exhaust gas passageway downstream of Catalyst O. Catalyst O corresponded to the above-mentioned NOx adsorbing and reducing catalyst.

Example 10

[0150] Catalyst P of Example 10 was produced by repeating the procedure of producing Catalyst L of Example 7 with the exception that a solution of magnesium acetate was used in place of the solution of cerium nitrate in the process of producing Catalyst L.

[0151] An exhaust gas purifying system of Example 10 was constituted by disposing Catalyst P in an exhaust gas passageway of an automotive internal combustion engine, and by disposing Catalyst A in the exhaust gas passageway downstream of Catalyst P. Catalyst P corresponded to the above-mentioned NOx adsorbing and reducing catalyst.

Example 11

[0152] Catalyst Q of Example 11 was produced by repeating the procedure of producing Catalyst L of Example 7 with the exception that an aqueous solution of cesium acetate was used in place of the aqueous solution of Ba acetate in the process of producing Catalyst L.

[0153] An exhaust gas purifying system of Example 11 was constituted by disposing Catalyst Q in an exhaust gas passageway of an automotive internal combustion engine, and by disposing Catalyst A in the exhaust gas passageway downstream of Catalyst Q. Catalyst Q corresponded to the above-mentioned NOx adsorbing and reducing catalyst.

Example 12

[0154] Catalyst R of Example 12 was produced by repeating the procedure of producing Catalyst L of Example 7 with the exception that the amount of all noble metals was ½of that in the process of producing Catalyst L.

[0155] An exhaust gas purifying system of Example 12 was constituted by disposing Catalyst L in an exhaust gas passageway of an automotive internal combustion engine, and by disposing two Catalysts R in series in the exhaust gas passageway downstream of Catalyst L. Catalyst R corresponded to the above-mentioned NOx adsorbing and reducing catalyst.

Example 13

[0156] An exhaust gas purifying system of Example 13 was constituted by disposing Catalyst M in an exhaust gas passageway of an automotive internal combustion engine, and by disposing Catalyst L in the exhaust gas passageway downstream of Catalyst M. Additionally, Catalyst A was disposed in the exhaust gas passageway downstream of Catalyst L.

Evaluation of Performance of Exhaust Gas Purifying System (3)

[0157] An internal combustion engine having a displacement of 2000 cc was provided with an exhaust system to which the exhaust gas purifying catalyst of Examples 7 to 13 and Comparative Examples 5 and 6 was incorporated. An evaluation test of the exhaust gas purifying system was conducted as follows:

[0158] This engine was operated repeating a changing operational cycle (evaluation mode) including a first engine operation step of 30 seconds on air-fuel mixture having an air/fuel ratio of 20, a second engine operation step of 4 seconds on air-fuel mixture having an air/fuel ratio of 11.0, and a third engine operation step of 5 seconds on air-fuel mixture having an air/fuel ratio of 14.7. The first, second and third engine operation steps were successively made in the order named. Such engine operation was carried out in two manners one of which was to control the temperature (or the inlet temperature) of exhaust gas immediately upstream of the upstreammost-side catalyst in the exhaust gas purifying system at 300° C. and the other was to control the temperature (or the inlet temperature) of exhaust gas immediately upstream of the exhaust gas purifying system at 400° C. Additionally, the temperature (or the inlet temperature) of the downstreammost-side catalyst was controlled at 180° C. In the evaluation test for the exhaust gas purifying system of Comparative Example 5, the inlet temperature of the upstream-side catalyst was set at 180° C. In the evaluation test for the exhaust gas purifying system of Comparative Example 13, the inlet temperature of Catalyst L was set at 300° C. at one manner and at 400° C. at the other manner. During one changing operational cycle in each manner of the engine operation, a concentration (ppm) of an exhaust gas component (HC, CO, NOx) was measured at positions of the exhaust gas passageway upstream and downstream of the exhaust gas purifying system. Such measurement was conducted on the exhaust gas purifying catalyst in a state after a durability test discussed below, thereby calculating a conversion rate (%) shown in Table 1. The conversion rate (%) was calculated by [(the concentration of the gas component in the exhaust gas passageway upstream of the exhaust gas purifying system—the concentration of the gas component in the exhaust gas passageway downstream of the exhaust gas purifying system/the concentration of the gas component in the exhaust gas passageway upstream of the exhaust gas purifying system)×100].

[0159] The durability test was conducted as follows: The exhaust gas purifying system of Examples and Comparative Examples was provided in an exhaust system of an internal combustion engine having a displacement of 4400 cc. The engine was operated for 50 hours in which the temperature (or the inlet temperature) of exhaust gas immediately upstream of the exhaust gas purifying system was kept at 750° C.

[0160] The specification of the catalysts of the exhaust gas purifying systems of Examples 7 to 13 and Comparative Examples 5 and 6 are summarized and shown in Table 5. The results of the evaluation test on the exhaust gas purifying systems of Examples 7 to 13 and Comparative Examples 5 and 6 are shown in Table 6. TABLE 5 Upstream-side NOx Adsorbing and reducing catalyst Downstream-side Noble Carrier for NOx Adsorbing NOx Adsorbing Sample metals noble metals material and reducing catalyst Reference Example 7 Pt, Rh Ce-added Al₂O₃ Ba Pt/Rh-Ba Example 8 Pt, Rh Ti-added Al₂O₃ Ba Pt/Rh-Ba Example 9 Pt, Rh Zr-added Al₂O₃ Ba Pt/Rh-Ba Example 10 Pt, Rh Ce-added Al₂O₃ Mg Pt/Rh-Ba Example 11 Pt, Rh Ce-added Al₂O₃ Cs Pt/Rh-Ba Example 12 Pt, Rh Ce-added Al₂O₃ Ba Pt/Rh-Ba Noble metal Catalyst is disposed amount = ½ upstream of of that in upstream-side NOx Example 7 adsorbing and reducing catalyst. Example 13 Pt, Rh Ce-added Al₂O₃ Ba Pt/Rh-Ba Compar. Pt, Rh Ce-added Al₂O₃ Ba Pt/Rh-Ba Example 5 Compar. Pt, Rh Ce-added Al₂O₃ Pt/Rh-Ba Example 6

[0161] TABLE 6 Inlet temp. of upstream-side Nox ad. Inlet temp. of upstream-side Nox ad. and red. catalyst = 300° C. and red. catalyst = 400° C. NOx HC CO NOx HC CO conversion conversion conversion conversion conversion conversion Sample rate rate rate rate rate rate Example 7 98% 98% 98% 98% 99% 99% Example 8 98% 98% 98% 98% 99% 99% Example 9 98% 98% 98% 98% 99% 99% Example 10 98% 98% 98% 98% 99% 99% Example 11 98% 98% 98% 98% 99% 99% Example 12 97% 98% 98% 97% 99% 99% Example 13 99% 98% 98% 99% 99% 99% Compar. Example 5 50% 96% 95% 50% 98% 97% Compar. Example 6 55% 98% 98% 55% 98% 99%

[0162] The results of the evaluation test reveal that the exhaust gas purifying systems of Examples 7 to 13 are high in catalytic activity and exhibit high conversion performances for NOx even in a low exhaust gas temperature region while reconciling good HC and CO removing performance and high NOx removing performance, as compared with the exhaust gas purifying systems of Comparative Examples 5 and 6.

Example 14

[0163] A catalyst of Example 14 corresponded to Catalyst F which was obtained in Example 4. An evaluation test discussed after will be conducted on Catalyst F.

Example 15

[0164] Catalyst C which had been previously prepared was dipped in a solution of magnesium acetate having a certain concentration, and then was blown with air to remove the excessive solution under the action of air stream. Thereafter, Catalyst C coated with the solution was dried at 150° C. in air stream and then calcined at 400° C. in air atmosphere thereby obtaining Catalyst 1 which was formed with a coat layer having a weight of 205 g per one liter of the monolithic substrate. Catalyst 1 had a MA/PA ratio of 5.3.

Example 16

[0165] Catalyst C which had been previously prepared was dipped in a solution of barium acetate having a certain concentration, and then was blown with air to remove the excessive solution under the action of air stream. Thereafter, Catalyst C coated with the solution was dried at 150° C. in air stream and then calcined at 400° C. in air atmosphere thereby obtaining Catalyst 2 which was formed with a coat layer having a weight of 220 g per one liter of the monolithic substrate. Catalyst 2 had a MA/PA ratio of 5.6.

Example 17

[0166] Catalyst C which had been previously prepared was dipped in a mixture solution of a certain concentration which solution containing barium acetate and magnesium acetate in a weight ratio of 3:1. Then, Catalyst C was blown with air to remove the excessive mixture solution under the action of air stream. Thereafter, Catalyst C coated with the solution was dried at 150° C. in air stream and then calcined at 400° C. in air atmosphere thereby obtaining Catalyst 3 which was formed with a coat layer having a weight of 215 g per one liter of the monolithic substrate. Catalyst 3 had a MA/PA ratio of 7.1.

Example 18

[0167] Catalyst C which had been previously prepared was dipped in a solution of barium acetate having a certain concentration, and then was blown with air to remove the excessive solution under the action of air stream. Thereafter, Catalyst C coated with the solution was dried at 150° C. in air stream and then calcined at 400° C. in air atmosphere thereby obtaining Catalyst 4 which was formed with a coat layer having a weight of 205 g per one liter of the monolithic substrate. Catalyst 4 had a MA/PA ratio of 1.4.

Example 19

[0168] Catalyst C which had been previously prepared was dipped in a solution of sodium acetate having a certain concentration, and then was blown with air to remove the excessive solution under the action of air stream. Thereafter, Catalyst C coated with the solution was dried at 150° C. in air stream and then calcined at 400° C. in air atmosphere thereby obtaining Catalyst 5 which was formed with a coat layer having a weight of 210 g per one liter of the monolithic substrate. Catalyst 5 had a MA/PA ratio of 6.9.

Example 20

[0169] Catalyst C which had been previously prepared was dipped in a solution of potassium acetate having a certain concentration, and then was blown with air to remove the excessive solution under the action of air stream. Thereafter, Catalyst C coated with the solution was dried at 150° C. in air stream and then calcined at 400° C. in air atmosphere thereby obtaining Catalyst 6 which was formed with a coat layer having a weight of 215 g per one liter of the monolithic substrate. Catalyst 6 had a MA/PA ratio of 6.8.

Example 21

[0170] Catalyst C which had been previously prepared was dipped in a solution of cesium acetate having a certain concentration, and then was blown with air to remove the excessive solution under the action of air stream. Thereafter, Catalyst C coated with the solution was dried at 150° C. in air stream and then calcined at 400° C. in air atmosphere thereby obtaining Catalyst 7 which was formed with a coat layer having a weight of 230 g per one liter of the monolithic substrate. Catalyst 7 had a MA/PA ratio of 4.6.

Example 22

[0171] Catalyst C which had been previously prepared was dipped in a solution of calcium acetate having a certain concentration, and then was blown with air to remove the excessive solution under the action of air stream. Thereafter, Catalyst C coated with the solution was dried at 150° C. in air stream and then calcined at 400° C. in air atmosphere thereby obtaining Catalyst 8 which was formed with a coat layer having a weight of 210 g per one liter of the monolithic substrate. Catalyst 8 had a MA/PA ratio of 7.6.

Example 23

[0172] Catalyst B which had been previously prepared was dipped in a solution of magnesium acetate having a certain concentration, and then was blown with air to remove the excessive solution under the action of air stream. Thereafter, Catalyst B coated with the solution was dried at 150° C. in air stream and then calcined at 400° C. in air atmosphere thereby obtaining Catalyst 9 which was formed with a coat layer having a weight of 210 g per one liter of the monolithic substrate. Catalyst 9 had a MA/PA ratio of 8.5.

Example 24

[0173] Catalyst D which had been previously prepared was dipped in a solution of magnesium acetate having a certain concentration, and then was blown with air to remove the excessive solution under the action of air stream. Thereafter, Catalyst D coated with the solution was dried at 150° C. in air stream and then calcined at 400° C. in air atmosphere thereby obtaining Catalyst 10 which was formed with a coat layer having a weight of 210 g per one liter of the monolithic substrate. Catalyst 10 had a MA/PA ratio of 8.5.

Example 25

[0174] Catalyst C which had been previously prepared was dipped in a solution of barium acetate having a certain concentration, and then was blown with air to remove the excessive solution under the action of air stream. Thereafter, Catalyst C coated with the solution was dried at 150° C. in air stream and then calcined at 400° C. in air atmosphere thereby obtaining Catalyst 11 which was formed with a coat layer having a weight of 250 g per one liter of the monolithic substrate. Catalyst 11 had a MA/PA ratio of 13.9.

Example 26

[0175] Catalyst C which had been previously prepared was dipped in a solution of barium acetate having a certain concentration, and then was blown with air to remove the excessive solution under the action of air stream. Thereafter, Catalyst C coated with the solution was dried at 150° C. in air stream and then calcined at 400° C. in air atmosphere thereby obtaining Catalyst 12 which was formed with a coat layer having a weight of 201 g per one liter of the monolithic substrate. Catalyst 12 had a MA/PA ratio of 0.3.

Example 27

[0176] Catalyst C which had been previously prepared was dipped in a solution of barium acetate having a certain concentration, and then was blown with air to remove the excessive solution under the action of air stream. Thereafter, Catalyst C coated with the solution was dried at 150° C. in air stream and then calcined at 400° C. in air atmosphere thereby obtaining Catalyst 13 which was formed with a coat layer having a weight of 211 g per one liter of the monolithic substrate. Catalyst 13 had a MA/PA ratio of 3.1.

Example 28

[0177] Catalyst C which had been previously prepared was dipped in a solution of barium acetate having a certain concentration, and then was blown with air to remove the excessive solution under the action of air stream. Thereafter, Catalyst C coated with the solution was dried at 150° C. in air stream and then calcined at 400° C. in air atmosphere thereby obtaining Catalyst 14 which was formed with a coat layer having a weight of 209 g per one liter of the monolithic substrate. Catalyst 14 had a MA/PA ratio of 9.5.

Evaluation of Performance of Exhaust Gas Purifying Catalyst

[0178] Tests for evaluation of catalytic activity were conducted on the catalysts of Examples 14 to 28 for exhaust gas purifying. The evaluation test for each catalyst was automatically carried out as follows: The catalyst was disposed in a gas passageway. Two laboratory model gases (lean gas and rich gas) simulating respectively actual lean exhaust gas and rich exhaust gas from an automotive internal combustion engine were flown alternately through the gas passage and the catalyst at a space velocity (SV) of 60000 Hr⁻¹, in which the temperature (or the inlet temperature) of the lean or rich gas immediately upstream of the catalyst was set at 200° C. It will be understood that the space velocity (SV) is represented by “flow amount (liter/hour) of the model gas/volume (liter) of the catalyst (or the substrate)”. In these tests, a concentration (ppm) of a gas component (CO) was measured at positions of the gas passageway upstream and downstream of the catalyst. Such measurement was made on the catalyst in a state after completion of a durability test discussed below, thereby calculating a conversion rate (%) or CO decreasing performance at a so-called rich spike (rich air/fuel ratio) as shown in Table 7. The conversion rate (%) was calculated by [(the concentration of the gas component in the gas passageway upstream of the exhaust gas purifying system—the concentration of the gas component in the gas passageway downstream of the catalyst/the concentration of the gas component in gas passageway upstream of the catalyst)×100].

[0179] The durability test was conducted on each of the catalysts of Examples 14 to 28 as follows: The catalyst was disposed in the exhaust gas passageway of an exhaust system of an internal combustion engine. The engine was operated for 30 hours in which the temperature (or the inlet temperature) of exhaust gas immediately upstream of the catalyst was kept at 700° C. In this connection, the above test was for the purpose of measuring deterioration of the CO decreasing catalyst upon being subjected to the durability test.

[0180] The two model gases (lean gas and rich gas) used in the above evaluation test had the following compositions:

[0181] The lean gas:

[0182] O₂=10 vol %

[0183] H₂O=10 vol %

[0184] NOx=300 ppm

[0185] N₂=balance

[0186] The rich gas:

[0187] H₂=2 vol %

[0188] CO=7 vol %

[0189] C₃H₆=1000 ppm C (calculated as carbon)

[0190] NOx=100 ppm

[0191] H₂O=10 vol %

[0192] N₂=balance

[0193] The results of the evaluation tests for the catalysts of Examples 14 to 28 are shown in Table 7 together with the compositions of the catalysts. TABLE 7 Co conversion Content of rate at rich element Sample air/fuel ratio Catalyst Element MA/MP ratio (g/l of substrate) Example 14 83% F Mg 7.6 7.1 Example 15 84% 1 Mg 5.3 5.0 Example 16 79% 2 Ba 5.6 20.0 Example 17 77% 3 Ba/Mg = 3/1 7.1 15.0 Example 18 78% 4 Ba 1.4 5.0 Example 19 73% 5 Na 6.9 10.0 Example 20 72% 6 K 6.8 15.0 Example 21 71% 7 Cs 4.5 30.0 Example 22 72% 8 Ca 7.6 10.0 Example 23 88% 9 Mg + Ce 8.5 8.0 Example 24 85% 10 Mg + Zr 8.5 8.0 Example 25 61% 11 Ba 13.9 50 Example 26 72% 12 Ba 0.3 1 Example 27 77% 13 Ba 3.1 11 Example 28 80% 14 Mg 9.5 9

[0194] The results of the evaluation test reveal that the exhaust gas purifying catalysts of Examples 14 to 28 are high in catalytic activity and exhibit high CO removing performances even in a low exhaust gas temperature range while reconciling high CO removing performance and high HC removing performance.

[0195] As apparent from the above, according to the present invention, the concentrations of the gas components of exhaust gas is regulated and/or adsorption and release of NOx are regulated in accordance with the temperature of exhaust gas. Accordingly, NOx in the oxygen-excessive region of exhaust gas discharged from the engine can be effectively reduced or removed, and/or CO and HC can be effectively oxidized or removed even if exhaust gas is in a low temperature range. Additionally, it is achieved to provide the exhaust gas purifying system which exhibits a high NOx removing efficiency throughout a wide range of exhaust gas temperature, particularly in the low exhaust gas temperature range.

[0196] The entire contents of Japanese Patent Applications P2000-283961 (filed Sep. 19, 2000) and P2001-282771 (filed Sep. 18, 2001) are incorporated herein by reference.

[0197] Although the invention has been described above by reference to certain embodiments and examples of the invention, the invention is not limited to the embodiments and examples described above. Modifications and variations of the embodiments and examples described above will occur to those skilled in the art, in light of the above teachings. The scope of the invention is defined with reference to the following claims. 

What is claimed is:
 1. An exhaust gas purifying system for an internal combustion engine, comprising: an exhaust gas component concentration regulating device disposed in an exhaust gas passageway of the engine, for regulating concentrations of gas components in exhaust gas discharged from the engine such that the concentrations of carbon monoxide and hydrogen are respectively not more than 2.0% by volume and not less than 0.5 by volume and such that a volume concentration ratio of [hydrogen/carbon monoxide] is not smaller than 0.5, in a first exhaust gas condition in which air/fuel ratio of exhaust gas is within a range of from a rich value and a stoichiometric value; and a NOx adsorbing and reducing catalyst disposed in the exhaust gas passageway downstream of said exhaust gas component concentration regulating device, for adsorbing nitrogen oxides in a second exhaust gas condition in which the air/fuel ratio of exhaust gas is at a lean value, and reducing the nitrogen oxides into nitrogen in the first exhaust gas condition.
 2. An exhaust gas purifying system as claimed in claim 1, wherein said exhaust gas component concentration regulating device is a rich exhaust gas CO and H₂ regulating catalyst for regulating a concentration of CO and H₂ in rich exhaust gas which has an air/fuel ratio richer than a stoichiometric value.
 3. An exhaust gas purifying system as claimed in claim 2, wherein the rich exhaust gas CO and H₂ regulating catalyst is a shift reaction catalyst for promoting a reaction represented by the following chemical formula: CO+H₂O→CO₂+H₂
 4. An exhaust gas purifying system as claimed in claim 3, wherein the shift reaction catalyst contains platinum, rhodium and a cerium compound carried on a substrate, the cerium compound being in an amount ranging from 5 to 100 g per one liter of the substrate, the amount being calculated as cerium oxide.
 5. An exhaust gas purifying system as claimed in claim 2, wherein the rich exhaust gas CO and H₂ regulating catalyst is a H₂ low consumption catalyst which is adapted to promote consumption of CO over that of H₂ in the rich exhaust gas.
 6. An exhaust gas purifying system as claimed in claim 5, wherein the H₂ low consumption catalyst contains at least one selected from the group consisting of platinum, rhodium, a titanium compound and a zirconium compound.
 7. An exhaust gas purifying system as claimed in claim 2, wherein the rich exhaust gas CO and H₂ regulating catalyst is a CO decreasing catalyst for decreasing a concentration of CO in the rich exhaust gas.
 8. An exhaust gas purifying system as claimed in claim 7, wherein the CO decreasing catalyst contains at least one noble metal selected from the group consisting of platinum, palladium and rhodium, and at least one selected from the group consisting of sodium, potassium, rubidium, cesium, magnesium, calcium, strontium and barium which are in form of compound.
 9. An exhaust gas purifying system as claimed in claim 8, wherein the CO decreasing catalyst contains at least one noble metal selected from the group consisting of platinum, palladium and rhodium in a number of moles of MP, and at least one selected from the group consisting of sodium, potassium, rubidium, cesium, magnesium, calcium, strontium and barium in a number of moles of MA, a ratio MA/MP being within a range of 0.1≦MA/MP≦10.0.
 10. An exhaust gas purifying system as claimed in claim 8, wherein the CO decreasing catalyst contains at least one noble metal selected from the group consisting of platinum, palladium and rhodium in a number of moles of MP, and at least one selected from the group consisting of sodium, potassium, rubidium, cesium, magnesium, calcium, strontium and barium in a number of moles of MA, a ratio MA/MP being within a range of 3.0≦MA/MP≦7.0.
 11. An exhaust gas purifying system as claimed in claim 8, wherein the CO decreasing catalyst contains at least one selected from the group consisting of sodium, potassium, rubidium, cesium, magnesium, calcium, strontium and barium which are in form of compound, in an amount ranging from 0.1 to 30 g per one liter of a substrate.
 12. An exhaust gas purifying system as claimed in claim 7, wherein the CO decreasing catalyst contains cerium.
 13. An exhaust gas purifying system as claimed in claim 8, wherein the CO decreasing catalyst contains zirconium.
 14. An exhaust gas purifying system as claimed in claim 3, wherein the rich exhaust gas CO and H₂ regulating catalyst is at least one selected from the group consisting of a shift reaction catalyst for promoting for promoting a reaction represented by the following chemical formula: CO+H₂O→CO₂+H₂ the shift reaction catalyst adapted to regulate the concentrations of carbon monoxide and hydrogen in the rich exhaust gas, a H₂ low consumption catalyst which is adapted to promote consumption of CO over that of H₂ in the rich exhaust gas, and a CO decreasing catalyst for decreasing the concentration of CO in the rich exhaust gas.
 15. An exhaust gas purifying system as claimed in claim 1, wherein said exhaust gas component concentration regulating device is a upstream-side NOx adsorbing and reducing catalyst for regulating concentrations of carbon monoxide and hydrogen in the first exhaust gas condition.
 16. An exhaust gas purifying system as claimed in claim 15, wherein temperature of said upstream-side NOx adsorbing and reducing catalyst is controlled within a range of from 200 to 500° C.
 17. An exhaust gas purifying system as claimed in claim 15, further comprising a catalyst condition detecting device for detecting a condition of said upstream-side NOx adsorbing and reducing catalyst, and a temperature control device for controlling at least one of a temperature of said upstream-side NOx adsorbing and reducing catalyst and a temperature of exhaust gas to flow into said upstream-side NOx adsorbing and reducing catalyst.
 18. An exhaust gas purifying system as claimed in claim 15, wherein said temperature control device includes a temperature raising device including at least one of a transmission ratio temperature raising device, a spark timing temperature raising device, a secondary fuel temperature raising device, a secondary air temperature raising device, a first electric heater for heating exhaust gas, and a second electric heater for heating a catalyst, the transmission ratio temperature raising device being adapted to increase a transmission ratio of an automatic transmission provided to the internal combustion engine so as to raise a temperature of exhaust gas of the engine, the spark timing temperature raising device being adapted to retard the spark timing of the engine relative to that during a normal engine operation so as to raise a temperature of exhaust gas of the engine, the secondary fuel temperature raising device being adapted to supply fuel into the exhaust gas passageway upstream of said upstream-side NOx adsorbing and reducing catalyst so as to raise a temperature of exhaust gas to be supplied to said upstream-side NOx adsorbing and reducing catalyst, the secondary air temperature raising device being adapted to supply air into said upstream-side NOx adsorbing and reducing catalyst so as to raise the temperature of said upstream-side NOx adsorbing and reducing catalyst, the first electric heater being adapted to heat exhaust gas to flow into said upstream-side NOx adsorbing and reducing catalyst, the second electric heater being adapted to heat said upstream-side NOx adsorbing and reducing catalyst.
 19. An exhaust gas purifying system as claimed in claim 15, wherein said temperature control device includes a temperature lowering device including a cooling device adapted to inject a cooling agent to the exhaust gas passageway upstream of said upstream-side NOx adsorbing and reducing catalyst.
 20. An exhaust gas purifying system as claimed in claim 10, wherein said upstream-side NOx adsorbing and reducing catalyst contains at least one of platinum and rhodium, and a cerium compound which are carried on a substrate, the cerium compound being in an amount ranging from 5 to 100 g per one liter of the substrate, the amount being calculated as cerium oxide.
 21. An exhaust gas purifying system as claimed in claim 10, wherein said upstream-side NOx adsorbing and reducing catalyst contains at lease one of platinum and rhodium, and at least one of titanium and zirconium compound.
 22. An exhaust gas purifying system for an internal combustion engine, comprising: a NOx adsorbing catalyst disposed in an exhaust gas passageway of the engine, said NOx adsorbing catalyst adapted to adsorb and release nitrogen oxides in accordance with a temperature of exhaust gas, said NOx adsorbing catalyst adapted to adsorb nitrogen oxides in exhaust gas in a first exhaust gas temperature range in which exhaust gas has a temperature ranging from a level at engine starting to 140° C. and release the adsorbed nitrogen oxides in a second exhaust gas temperature range in which exhaust gas has a temperature of not lower than 200° C., exhaust gas discharged from said NOx adsorbing catalyst having a volume concentration ratio of [nitrogen oxides/carbon monoxide] which is not larger than 0.3 in the first exhaust gas temperature range; and an oxidizing catalyst disposed in the exhaust gas passageway downstream of said NOx adsorbing catalyst, for oxidizing oxidizable components in exhaust gas discharged from said NOx adsorbing catalyst.
 23. An exhaust gas purifying system as claimed in claim 22, wherein said NOx adsorbing catalyst includes a substrate, and platinum carried on the substrate, in an amount ranging from 0.1 to 10 g per one liter of the substrate.
 24. An exhaust gas purifying system as claimed in claim 23, wherein said NOx adsorbing catalyst contains at least one of alkali metal, alkaline-earth metal and rare earth element, carried on the substrate in an amount ranging from 1 to 50 g per one liter of the substrate.
 25. An exhaust gas purifying system as claimed in claim 22, wherein said NOx adsorbing catalyst is adapted to adsorb nitrogen oxides in exhaust gas in a third exhaust gas temperature range in which exhaust gas has a temperature ranging from 100 to 140° C.
 26. An exhaust gas purifying system as claimed in claim 22, wherein said oxidizing catalyst includes a substrate, and platinum carried on the substrate, in an amount ranging from 0.5 to 20 g per one liter of the substrate.
 27. An exhaust gas purifying system as claimed in claim 26, wherein said oxidizing catalyst includes zeolite carried on the substrate, in an amount ranging form 10 to 100 g per one liter of the substrate.
 28. A NOx adsorbing catalyst used in an exhaust gas purifying system for an internal combustion engine, the NOx adsorbing catalyst being disposed in an exhaust gas passageway of the engine and adapted to adsorb and release nitrogen oxides in exhaust gas in accordance with a temperature of exhaust gas, said exhaust gas purifying system including an oxidizing catalyst in the exhaust gas passageway downstream of said NOx adsorbing catalyst, for oxidizing oxidizable components in exhaust gas discharged from said NOx adsorbing catalyst, wherein said NOx adsorbing catalyst is adapted to adsorb nitrogen oxides in exhaust gas in a first exhaust gas temperature range in which exhaust gas has a temperature ranging from a level at engine starting to 140° C. and release the adsorbed nitrogen oxides in a second exhaust gas temperature condition in which exhaust gas has a temperature of not lower than 200° C., exhaust gas discharged from said NOx adsorbing catalyst having a volume concentration ratio of [nitrogen oxides/carbon monoxide] which is not larger than 0.3 in the first exhaust gas temperature range.
 29. An exhaust gas purifying system for an internal combustion engine, comprising an exhaust gas component regulating device disposed in an exhaust gas passageway of the engine, at least one of a NOx adsorbing and reducing catalyst and a NOx adsorbing catalyst disposed in the exhaust gas passageway downstream of said exhaust gas component regulating device, and an oxidizing catalyst in the exhaust gas passageway downstream of the at least one of said NOx adsorbing and reducing catalyst and said NOx adsorbing catalyst, said exhaust gas component concentration regulating device being adapted to regulate concentrations of gas components in exhaust gas discharged from the engine such that the concentrations of carbon monoxide and hydrogen are respectively not more than 2.0% by volume and not less than 0.5% by volume and such that a volume concentration ratio of [hydrogen/carbon monoxide] is not smaller than 0.5, in a first exhaust gas condition in which air/fuel ratio of exhaust gas is within a range of from a rich value and a stoichiometric value, said NOx adsorbing and reducing catalyst being adapted to adsorb nitrogen oxides in a second exhaust gas condition in which the air/fuel ratio of exhaust gas is at a lean value, and reducing the nitrogen oxides into nitrogen in the first exhaust gas condition, said NOx adsorbing catalyst being adapted to adsorb and release nitrogen oxides in accordance with a temperature of exhaust gas, said NOx adsorbing catalyst adapted to adsorb nitrogen oxides in exhaust gas in a first exhaust gas temperature range in which exhaust gas has a temperature ranging from a level at engine starting to 140° C. and release the adsorbed nitrogen oxides in a second exhaust gas temperature range in which exhaust gas has a temperature of not lower than 200° C., exhaust gas discharged from said NOx adsorbing catalyst having a volume concentration ratio of [nitrogen oxides/carbon monoxide] which is not larger than 0.3 in the first exhaust gas temperature range, said oxidizing catalyst being adapted to oxidize oxidizable components in exhaust gas discharged from the at least one of said NOx adsorbing and reducing catalyst and said NOx adsorbing catalyst. 